Chapter XII: The Organs of Excretion (11)

They were made, in the first instance, to complete a chapter in my work on comparative embryology; and, partly owing to the press of other engagements, but still more to the difficulty of procuring material, my observations are confined to the two British species of the genus _Scyllium_, viz. _Sc. stellare_ and _Sc. canicula_; yet I venture to believe that the results at which I have arrived are not wholly without interest.

Before dealing with the development of the skeleton of the fin, it will be convenient to describe with great brevity the structure of the pectoral and pelvic fins of the adult. The pectoral fins consist of broad plates inserted horizontally on the sides of the body; so that in each there may be distinguished a dorsal and a ventral surface, and an anterior and a posterior border. Their shape may best be gathered from the woodcut (fig. 1); and it is to be especially noted that the narrowest part of the fin is the base, where it is attached to the side of the body. The cartilaginous skeleton only occupies a small zone at the base of the fin, the remainder being formed of a fringe supported by radiately arranged horny fibres[493].

Footnote 493: The horny fibres are mesoblastic products; they
are formed, in the first instance, as extremely delicate
fibrils on the inner side of the membrane separating the
epiblast from the mesoblast.

Pectoral fins and girdle of an adult of _Scyllium canicula_ (natural size, seen from behind and above).

_co._ Coracoid. _sc._ scapula. _pp._ propterygium. _mep._ mesopterygium. _mp._ metapterygium. _fn._ part of fin supported by horny fibre.]

Right pelvic fin and part of pelvic girdle of an adult female of _Scyllium canicula_ (natural size).

_il._ iliac process. _pn._ pubic process, cut across below. _bp._ basipterygium. _af._ anterior cartilaginous fin-ray articulated to pelvic girdle. _fn._ part of fin supported by horny fibres.]

The true skeleton consists of three basal pieces articulating with the pectoral girdle; on the outer side of which there is a series of more or less segmented cartilaginous fin-rays. Of the basal cartilages one (_pp_) is anterior, a second (_mep_) is placed in the middle, and a third is posterior (_mp_). They have been named by Gegenbaur the _propterygium_, the _mesopterygium_, and the _metapterygium_; and these names are now generally adopted.

The metapterygium is by far the most important of the three, and in _Scyllium canicula_ supports 12 or 13 rays[494]. It forms a large part of the posterior boundary of the fin, and bears rays only on its _anterior_ border.

Footnote 494: In one example where the metapterygium had 13
rays the mesopterygium had only 2 rays.

The mesopterygium supports 2 or 3 rays, in the basal parts of which the segmentation into distinct rays is imperfect; and the propterygium supports only a single ray.

The pelvic fins are horizontally placed, like the pectoral fins, but differ from the latter in nearly meeting each other along the median ventral line of the body. They also differ from the pectoral fins in having a relatively much broader base of attachment to the sides of the body. Their cartilaginous skeleton (woodcut, fig. 2) consists of a basal bar, placed parallel to the base of the fin, and articulated in front with the pelvic girdle.

On its outer border it articulates with a series of cartilaginous fin-rays. I shall call the basal bar the basipterygium. The rays which it bears are most of them less segmented than those of the pectoral fin, being only divided into two; and the posterior ray, which is placed in the free posterior border of the fin, continues the axis of the basipterygium. In the male it is modified in connection with the so-called clasper.

The anterior fin-ray of the pelvic fin, which is broader than the other rays, articulates directly with the pelvic girdle, instead of with the basipterygium. This ray, in the female of _Scyllium canicula_ and in the male of _Scyllium catulus_ (Gegenbaur), is peculiar in the fact that its distal segment is longitudinally divided into two or more pieces, instead of being single as is the case with the remaining rays. It is probably equivalent to two of the posterior rays.

_Development of the paired Fins._--The first rudiments of the limbs appear in _Scyllium_, as in other fishes, as slight longitudinal ridge-like thickenings of the epiblast, which closely resemble the first rudiments of the unpaired fins.

These ridges are two in number on each side--an anterior immediately behind the last visceral fold, and a posterior on the level of the cloaca. In most Fishes they are in no way connected; but in some Elasmobranch embryos, more especially in that of _Torpedo_, they are connected together at their first development by a line of columnar epiblast cells. This connecting line of columnar epiblast, however, is a very transitory structure. The rudimentary fins soon become more prominent, consisting of a projecting ridge both of epiblast and mesoblast, at the outer edge of which is a fold of epiblast only, which soon reaches considerable dimensions. At a later stage the mesoblast penetrates into this fold, and the fin becomes a simple ridge of mesoblast covered by epiblast. The pectoral fins are at first considerably ahead of the pelvic fins in development.

The direction of the original epithelial line which connected the two fins of each side is nearly, though not quite, longitudinal, sloping somewhat obliquely ventralwards. It thus comes about that the attachment of each pair of limbs is somewhat on a slant, and that the pelvic pair nearly meet each other in the median ventral line shortly behind the anus.

The embryonic muscle-plates, as I have elsewhere shewn, grow into the bases of the fins; and the cells derived from these ingrowths, which are placed on the dorsal and ventral surfaces in immediate contact with the epiblast, probably give rise to the dorsal and ventral muscular layers of the limb, which are shewn in section in Plate 33, fig. 1, _m_, and in Plate 33, fig. 7, _m_.

The cartilaginous skeleton of the limbs is developed in the indifferent mesoblast cells between the two layers of muscles. Its early development in both the pectoral and the pelvic fins is very similar. When first visible it differs histologically from the adjacent mesoblast simply in the fact of its cells being more concentrated; while its boundary is not sharply marked.

At this stage it can only be studied by means of sections. It arises simultaneously and continuously with the pectoral and pelvic girdles, and consists, in both fins, of a bar springing at right angles from the posterior side of the pectoral or pelvic girdle, and running parallel to the long axis of the body along the base of the fin. The outer side of this bar is continued into a thin plate, which extends into the fin.

The structure of the skeleton of the fin slightly after its first differentiation will be best understood from Plate 33, fig. 1, and Plate 33, fig. 7. These figures represent transverse sections through the pelvic and pectoral fins of the same embryo on the same scale. The basal bar is seen at _bp_, and the plate at this stage (which is considerably later than the first differentiation) already partially segmented into rays at _br_. Outside the region of the cartilaginous plate is seen the fringe with the horny fibres (_h.f._); and dorsally and ventrally to the cartilaginous skeleton are seen the already well-differentiated muscles (_m_).

The pectoral fin is shewn in horizontal section in Plate 33, fig. 6, at a somewhat earlier stage than that to which the transverse sections belong. The pectoral girdle (_p.g._) is cut transversely, and is seen to be perfectly continuous with the basal bar (_vp_) of the fin. A similar continuity between the basal bar of the pelvic fin and the pelvic girdle is shewn in Plate 33, fig. 2, at a somewhat later stage. The plate continuous with the basal bar of the fin is at first, to a considerable extent in the pectoral, and to some extent in the pelvic fin, a continuous lamina, which subsequently segments into rays. In the parts of the plate which eventually form distinct rays, however, almost from the first the cells are more concentrated than in those parts which will form the tissue between the rays; and I am not inclined to lay any stress whatever upon the fact of the cartilaginous fin-rays being primitively part of a continuous lamina, but regard it as a secondary phenomenon, dependent on the mode of conversion of embryonic mesoblast cells into cartilage. In all cases the separation into distinct rays is to a large extent completed before the tissue of which the plates are formed is sufficiently differentiated to be called cartilage by an histologist.

The general position of the fins in relation to the body, and their relative sizes, may be gathered from Plate 33, figs. 4 and 5, which represent transverse sections of the same embryo as that from which the transverse sections shewing the fin on a larger scale were taken.

During the first stage of its development the skeleton of both fins may thus be described as consisting of _a longitudinal bar running along the base of the fin, and giving off at right angles series of rays which pass into the fin_. The longitudinal bar may be called the basipterygium; and it is continuous in front with the pectoral or pelvic girdle, as the case may be.

The further development of the primitive skeleton is different in the case of the two fins.

_The Pelvic Fin._--The changes in the pelvic fin are comparatively slight. Plate 33, fig. 2, is a representation of the fin and its skeleton in a female of _Scyllium stellare_ shortly after the primitive tissue is converted into cartilage, but while it is still so soft as to require the very greatest care in dissection. The fin itself forms a simple projection of the side of the body. The skeleton consists of a basipterygium (_bp_), continuous in front with the pelvic girdle. To the outer side of the basipterygium a series of cartilaginous fin-rays are attached--the posterior ray forming a direct prolongation of the basipterygium, while the anterior ray is united rather with the pelvic girdle than with the basipterygium. All the cartilaginous fin-rays except the first are completely continuous with the basipterygium, their structure in section being hardly different from that shewn in Plate 33, fig. 1.

The external form of the fin does not change very greatly in the course of the further development; but the hinder part of the attached border is, to some extent, separated off from the wall of the body, and becomes the posterior border of the adult fin. With the exception of a certain amount of segmentation in the rays, the character of the skeleton remains almost as in the embryo. The changes which take place are illustrated by Plate 33, fig. 3, shewing the fin of a young male of _Scyllium stellare_. The basipterygium has become somewhat thicker, but is still continuous in front with the pelvic girdle, and otherwise retains its earlier characters. The cartilaginous fin-rays have now become segmented off from it and from the pelvic girdle, the posterior end of the basipterygial bar being segmented off as the terminal ray.

The anterior ray is directly articulated with the pelvic girdle, and the remaining rays continue articulated with the basipterygium. Some of the latter are partially segmented.

As may be gathered by comparing the figure of the fin at the stage just described with that of the adult fin (woodcut, fig. 2), the remaining changes are very slight. The most important is the segmentation of the basipterygial bar from the pelvic girdle.

The pelvic fin thus retains in all essential points its primitive structure.

_The Pectoral Fin._--The earliest stage of the pectoral fin differs, as I have shewn, from that of the pelvic fin only in minor points (Pl. 33, fig. 6). There is the same longitudinal or basipterygial bar (_bp_), to which the fin-rays are attached, which is continuous in front with the pectoral girdle (_pg_). The changes which take place in the course of the further development, however, are very much more considerable in the case of the pectoral than in that of the pelvic fin.

The most important change in the external form of the fin is caused by a reduction in the length of its attachment to the body. At first (Pl. 33, fig. 6), the base of the fin is as long as the greatest breadth of the fin; but it gradually becomes shortened by being constricted off from the body at its hinder end. In connection with this process the posterior end of the basipterygial bar is gradually rotated outwards, its anterior end remaining attached to the pectoral girdle. In this way this bar comes to form the posterior border of the skeleton of the fin (Pl. 33, figs. 8 and 9), constituting the metapterygium (_mp_). It becomes eventually segmented off from the pectoral girdle, simply articulating with its hinder edge.

The plate of cartilage, which is continued outwards from the basipterygium, or, as we may now call it, the metapterygium, into the fin, is not nearly so completely divided up into fin-rays as the homologous part of the pelvic fin; and this is especially the case with the basal part of the plate. This basal part becomes, in fact, at first only divided into two parts (Pl. 33, fig. 8)--a small anterior part at the front end (_me.p_), and a larger posterior along the base of the metapterygium (_mp_); and these two parts are not completely segmented from each other. The anterior part directly joins the pectoral girdle at its base, resembling in this respect the anterior fin-ray of the pelvic girdle. It constitutes the (at this stage undivided) rudiment of the mesopterygium and propterygium of Gegenbaur. It bears in my specimen of this age four fin-rays at its extremity, the anterior not being well marked. The remaining fin-rays are prolongations outwards of the edge of the plate continuous with the metapterygium. These rays are at the stage figured more or less transversely segmented; but at their outer edge they are united together by a nearly continuous rim of cartilage. The spaces between the fin-rays are relatively considerably larger than in the adult.

The further changes in the cartilages of the pectoral limb are, morphologically speaking, not important, and are easily understood by reference to Pl. 33, fig. 9 (representing the skeleton of the limb of a nearly ripe embryo). The front end of the anterior basal cartilage becomes segmented off as a propterygium (_pp_), bearing a single fin-ray, leaving the remainder of the cartilage as a mesopterygium (_mes_). The remainder of the now considerably segmented fin-rays are borne by the metapterygium.

* * * * *

_General Conclusions._--From the above observations, conclusions of a positive kind may be drawn as to the primitive structure of the skeleton; and the observations have also, it appears to me, important bearings on the theories of my predecessors in this line of investigation.

The most obvious of the positive conclusions is to the effect that the embryonic skeleton of the paired fins consists of a series of parallel rays similar to those of the unpaired fins. These rays support the soft parts of the fins, which have the form of a longitudinal ridge; and they are continuous at their base with a longitudinal bar. This bar, from its position at the base of the fin, can clearly never have been a median axis with the rays on both sides. It becomes the basipterygium in the pelvic fin, which retains its embryonic structure much more completely than the pectoral fin; and the metapterygium in the pectoral fin. The metapterygium of the pectoral fin is thus clearly homologous with the basipterygium of the pelvic fin, as originally supposed by Gegenbaur, and as has since been maintained by Mivart. The propterygium and mesopterygium are obviously relatively _unimportant_ parts of the skeleton as compared with the metapterygium.

My observations on the development of the skeleton of the fins certainly do not of themselves demonstrate that the paired fins are remnants of a once continuous lateral fin; but they support this view in that they shew the primitive skeleton of the fins to have exactly the character which might have been anticipated if the paired fins had originated from a continuous lateral fin. The longitudinal bar of the paired fins is believed by both Thacker and Mivart to be due to the coalescence of the bases of the primitively independent rays of which they believe the fin to have been originally composed. This view is probable enough in itself, and is rendered more so by the fact, pointed out by Mivart, that a longitudinal bar supporting the cartilaginous rays of unpaired fins is occasionally formed; but there is no trace in the embryo Scylliums of the bar in question being formed by the coalescence of rays, though the fact of its being perfectly continuous with the bases of the fin-rays is somewhat in favour of such coalescence.

Thacker and Mivart both hold that the pectoral and pelvic girdles are developed by ventral and dorsal growths of the anterior end of the longitudinal bar supporting the fin-rays.

There is, so far as I see, no theoretical objection to be taken to this view; and the fact of the pectoral and pelvic girdles originating continuously and long remaining united with the longitudinal bars of their respective fins is in favour of it rather than the reverse. The same may be said of the fact that the first part of each girdle to be formed is that in the neighbourhood of the longitudinal bar (basipterygium) of the fin, the dorsal and ventral prolongations being subsequent growths.

On the whole my observations do not throw much light on the theories of Thacker and Mivart as to the genesis of the skeleton of the paired fin; but, so far as they bear on the subject, they are distinctly favourable to those theories.

The main results of my observations appear to me to be decidedly adverse to the views recently put forward on the structure of the fin by Gegenbaur and Huxley, both of whom, as stated above, consider the primitive type of fin to be most nearly retained in _Ceratodus_, and to consist of a central multisegmented axis with numerous lateral rays.

Gegenbaur derives the Elasmobranch pectoral fin from a form which he calls the archipterygium, nearly like that of _Ceratodus_, with a median axis and two rows of rays--but holds that in addition to the rays attached to the median axis, which are alone found in _Ceratodus_, there were other rays directly articulated to the shoulder-girdle. He considers that in the Elasmobranch fin the majority of the lateral rays on the posterior (or median according to his view of the position of the limb) side have become aborted, and that the central axis is represented by the metapterygium; while the pro- and mesopterygium and their rays are, he believes, derived from those rays of the archipterygium which originally articulated directly with the shoulder-girdle.

This view appears to me to be absolutely negatived by the facts of development of the pectoral fin in _Scyllium_--not so much because the pectoral fin in this form is necessarily to be regarded as primitive, but because what Gegenbaur holds to be the primitive axis of the biserial fin is demonstrated to be really the base, and it is only in the adult that it is conceivable that a second set of lateral rays could have existed on the posterior side of the metapterygium. If Gegenbaur's view were correct, we should expect to find in the embryo, if anywhere, traces of the second set of lateral rays; but the fact is that, as may easily be seen by an inspection of figs. 6 and 7, such a second set of lateral rays could not possibly have existed in a type of fin like that found in the embryo. With this view of Gegenbaur's it appears to me that the theory held by this anatomist to the effect that the limbs are modified gill-arches also falls, in that his method of deriving the limbs from gill-arches ceases to be admissible, while it is not easy to see how a limb, formed on the type of the embryonic limb of Elasmobranchii, could be derived from a gill-arch with its branchial rays.

Gegenbaur's older view, that the Elasmobranch fin retains a primitive uniserial type, appears to me to be nearer the truth than his more recent view on this subject; though I hold the fundamental point established by the development of these parts in _Scyllium_ to be that the posterior border of the adult Elasmobranch pectoral fin is the primitive base-line,_i.e._line of attachment of the fin to the side of the body.

Huxley holds that the mesopterygium is the proximal piece of the axial skeleton of the limb of _Ceratodus_, and derives the Elasmobranch fin from that of _Ceratodus_ by the shortening of its axis and the coalescence of some of its elements. The entirely secondary character of the mesopterygium, and its total absence in the young embryo _Scyllium_, appear to me as conclusive against Huxley's view as the character of the embryonic fin is against that of Gegenbaur; and I should be much more inclined to hold that the fin of _Ceratodus_ has been derived from a fin like that of the Elasmobranchii by a series of steps similar to those which Huxley supposes to have led to the establishment of the Elasmobranch fin, but in exactly the reverse order.

There is one statement of Davidoff's which I cannot allow to pass without challenge. In comparing the skeletons of the paired and unpaired fins he is anxious to prove that the former are independent of the axial skeleton in their origin and that the latter have been segmented from the axial skeleton, and thus to shew that an homology between the two is impossible. In support of his view he states[495] that he has satisfied himself, from embryos of _Acanthias_ and _Scyllium_, that the rays of the unpaired fins _are undoubtedly products of the segmentation of the dorsal and ventral spinous processes_.

Footnote 495: _Loc. cit._ p. 514.

This statement is wholly unintelligible to me. From my examination of the development of the first dorsal and the anal fins of _Scyllium_ I find that their rays develop at a considerable distance from, and quite independently of, the neural and hæmal arches, and that they are at an early stage of development distinctly in a more advanced state of histological differentiation than the neural and hæmal arches of the same region. I have also found exactly the same in the embryos of _Lepidosteus_.

I have, in fact, no doubt that the skeleton of both the paired and the unpaired fins of Elasmobranchii and _Lepidosteus_ is in its development independent of the axial skeleton. The phylogenetic mode of origin of the skeleton both of the paired and of the unpaired fins cannot, however, be made out without further investigation.

EXPLANATION OF PLATE 33.[496]

Footnote 496: I employ here the same letters to indicate the
stages as in my "Monograph on Elasmobranch Fishes."

Fig. 1. Transverse section through the pelvic fin of an embryo of _Scyllium_ belonging to stage P{1}, magnified 50 diameters. _bp._ basipterygium. _br._ fin ray. _m._ muscle. _hf._ horny fibres supporting the peripheral part of the fin.

Fig. 2. Pelvic fin of a very young female embryo of _Scyllium stellare_, magnified 16 diameters. _bp._ basipterygium. _pu._ pubic process of pelvic girdle (cut across below). _il._ iliac process of pelvic girdle. _fo._ foramen.

Fig. 3. Pelvic fin of a young male embryo of _Scyllium stellare_, magnified 16 diameters. _bp._ basipterygium. _mo._ process of basipterygium continued into clasper. _il._ iliac process of pelvic girdle. _pu._ pubic section of pelvic girdle.

Fig. 4. Transverse section through the ventral part of the trunk of an embryo _Scyllium_ of stage P, in the region of the pectoral fins, to shew how the fins are attached to the body, magnified 18 diameters. _br._ cartilaginous fin-ray. _bp._ basipterygium. _m._ muscle of fin. _mp._ muscle-plate.

Fig. 5. Transverse section through the ventral part of the trunk of an embryo _Scyllium_ of stage P, in the region of the pelvic fin, on the same scale as fig. 4. _bp._ basipterygium. _br._ cartilaginous fin-rays. _m._ muscle of the fins. _mp._ muscle-plate.

Fig. 6. Pectoral fin of an embryo of _Scyllium canicula_, of a stage between O and P, in longitudinal and horizontal section (the skeleton of the fin was still in the condition of embryonic cartilage), magnified 36 diameters. _bp._ basipterygium (eventual metapterygium). _fr._ cartilaginous fin-rays. _pg._ pectoral girdle in transverse section. _fo._ foramen in pectoral girdle. _pe._ epithelium of peritoneal cavity.

Fig. 7. Transverse section through the pectoral fin of a _Scyllium_ embryo of stage P, magnified 50 diameters. _bp._ basipterygium. _br._ cartilaginous fin-ray. _m._ muscle. _hf._ horny fibres.

Fig. 8. Pectoral fin of an embryo of _Scyllium stellare_, magnified 16 diameters. _mp._ metapterygium (basipterygium of earlier stage). _me.p._ rudiment of future pro- and mesopterygium. _sc._ cut surface of a scapular process. _cr._ coracoid process. _fr._ foramen. _hf._ horny fibres.

Fig. 9. Skeleton of the pectoral fin and part of pectoral girdle of a nearly ripe embryo of _Scyllium stellare_, magnified 10 diameters. _mp._ metapterygium. _mes._ mesopterygium. _pp._ propterygium. _cr._ coracoid process.

XXI. ON THE EVOLUTION OF THE PLACENTA, AND ON THE POSSIBILITY OF EMPLOYING THE CHARACTERS OF THE PLACENTA IN THE CLASSIFICATION OF THE MAMMALIA[497].

Footnote 497: From the _Proceedings of the Zoological Society
of London_, 1881.

From Owen's observations on the Marsupials it is clear that the yolk-sack in this group plays an important (if not the most important) part, in absorbing the maternal nutriment destined for the foetus. The fact that in Marsupials both the yolk-sack and the allantois are concerned in rendering the chorion vascular, makes it _à priori_ probable that this was also the case in the primitive types of the Placentalia; and this deduction is supported by the fact that in the Rodentia, Insectivora, and Cheiroptera this peculiarity of the foetal membranes is actually found. In the primitive Placentalia it is also probable that from the discoidal allantoic region of the chorion simple foetal villi, like those of the Pig, projected into uterine crypts; but it is not certain how far the umbilical region of the chorion, which was no doubt vascular, may also have been villous. From such a primitive type of foetal membranes divergencies in various directions have given rise to the types of foetal membranes found at the present day.

In a general way it may be laid down that variations in any direction which tended to increase the absorbing capacities of the chorion would be advantageous. There are two obvious ways in which this might be done, viz. (1) by increasing the complexity of the foetal villi and maternal crypts over a limited area, (2) by increasing the area of the part of the chorion covered by the placental villi. Various combinations of the two processes would also, of course, be advantageous.

The most fundamental change which has taken place in all the existing Placentalia is the exclusion of the umbilical vesicle from any important function in the nutrition of the foetus.

The arrangement of the foetal parts in the Rodentia, Insectivora, and Cheiroptera may be directly derived from the primitive form by supposing the villi of the discoidal placental area to have become more complex, so as to form a deciduate discoidal placenta, while the yolk-sack still plays a part, though physiologically an unimportant part, in rendering the chorion vascular.

In the Carnivora, again, we have to start from the discoidal placenta, as evinced by the fact that in the growth of the placenta the allantoic region of the placenta is at first _discoidal_, and only becomes zonary at a later stage. A zonary deciduate placenta indicates an increase both in area and in complexity. The relative diminution of the breadth of the placental zone in late foetal life in the zonary placenta of the Carnivora is probably due to its being on the whole advantageous to secure the nutrition of the foetus by insuring a more intimate relation between the foetal and maternal parts, than by increasing their area of contact. The reason of this is not obvious, but, as shewn below, there are other cases where it is clear that a diminution in the area of the placenta has taken place, accompanied by an increase in the complexity of its villi.

The second type of differentiation from the primitive form of placenta is illustrated by the Lemuridæ, the Suidæ, and _Manis_. In all these cases the area of the placental villi appears to have increased so as to cover nearly the whole subzonal membrane, without the villi increasing to any great extent in complexity. From the diffused placenta covering the whole surface of the chorion, differentiations appear to have taken place in various directions. The placenta of Man and Apes, from its mode of ontogeny, is clearly derived from a diffused placenta (very probably similar to that of Lemurs) by a concentration of the foetal villi, which are originally spread over the whole chorion, to a disk-shaped area, and by an increase in their arborescence. Thus the discoidal placenta of Man has no connexion with, and ought not to be placed in, the same class as those of the Rodentia, Cheiroptera, and Insectivora.

The polycotyledonary forms of placenta are due to similar concentrations of the foetal villi of an originally diffused placenta.

In the Edentata we have a group with very varying types of placenta. Very probably these may all be differentiations within the group itself from a diffused placenta such as that found in _Manis_. The zonary placenta of _Orycteropus_ is capable of being easily derived from that of _Manis_ by the disappearance of the foetal villi at the two poles of the ovum. The small size of the umbilical vesicle in _Orycteropus_ indicates that its discoidal placenta is not, like that of the Carnivora, directly derived from a type with both allantoic and umbilical vascularization of the chorion. The discoidal and dome-shaped placentæ of the Armadillos, _Myrmecophaga_, and the Sloths may easily have been formed from a diffused placenta, just as the discoidal placenta of the Simiidæ and Hominidæ appears to have been formed from a diffused placenta like that of the Lemuridæ.

The presence of zonary placenta in _Hyrax_ and _Elephas_ does not necessarily afford any proof of affinity of these types with the Carnivora. A zonary placenta may be quite as easily derived from a diffused placenta as from a discoidal placenta; and the presence of two villous patches at the poles of the chorion in _Elephas_ very probably indicates that its placenta has been evolved from a diffused placenta.

Although it would not be wise to attempt to found a classification upon the placental characters alone, it may be worth while to make a few suggestions as to the affinities of the orders of Mammalia indicated by the structure of the placenta. We clearly, of course, have to start with forms which could not be grouped with any of the existing orders, but which might be called the Protoplacentalia. They probably had the primitive type of placenta described above: the nearest living representatives of the group are the Rodentia, Insectivora, and Cheiroptera. Before, however, these three groups had become distinctly differentiated, there must have branched off from the primitive stock the ancestors of the Lemuridæ, the Ungulata, and the Edentata.

It is obvious on general anatomical grounds that the Monkeys and Man are to be derived from a primitive Lemurian type; and with this conclusion the form of the placenta completely tallies. The primitive Edentata and Ungulata had no doubt a diffused placenta which was probably not very different from that of the primitive Lemurs; but how far these groups arose quite independently from the primitive stock, or whether they may have had a nearer common ancestor, cannot be decided from the structure of the placenta. The Carnivora were certainly an offshoot from the primitive placental type which was quite independent of the three groups just mentioned; but the character of the placenta of the Carnivora does not indicate at what stage in the evolution of the placental Mammalia a primitive type of Carnivora was first differentiated.

No important light is thrown by the placenta on the affinities of the Proboscidea, the Cetacea, or the Sirenia; but the character of the placenta in the latter group favours the view of their being related to the Ungulata.

XXII. ON THE STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS[498].
By F. M. BALFOUR and W. N. PARKER.

Footnote 498: From the _Philosophical Transactions of the
Royal Society_, 1882.

(With Plates 34-42.)

TABLE OF CONTENTS.

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