Chapter III: Development of the Ovum (2)

The cavity of the abdomen is therefore at first open to the vesicula umbilicalis or yelk, but this changes as the abdominal parietes begin to close in; in man and the mammalia merely a part of it, as above mentioned, forms the intestinal canal, whereas, in oviparous animals the whole of the yelk-bag enters the abdominal cavity, and serves for an early nutriment to the young animal. Another change connected with the serous or outer layer of the germinal membrane is the formation of the _amnion_. The foetal rudiment which from its shape has been called _carina_, now begins to be enveloped by a membrane of exceeding tenuity, forming a double covering upon it; the one which immediately invests the foetus is considered to form the future epidermis; the other, or outer fold, forms a loose sac around it, containing the liquor amnii. Whilst these changes are taking place in the serous layer of the germinal membrane, and whilst the intestinal canal, &c. are forming on the anterior surface of the embryo, which is turned towards the ovum, by means of the inner or mucous layer, equally important changes are now observed in the middle or vascular layer. "In forming this fold," says Dr. A. Thomson, "the mucous layer is reflected farthest inwards; the serous layer advances least, and the space between them, occupied by the vascular layer, is filled up by a dilated part of this layer, the rudiment of the heart." (_Op. cit._ p. 301.)

Whilst this rudimentary trace of the vascular system is making its appearance, minute vessels are seen ramifying over the vesicula umbilicalis, forming, according to Baer's observations, a reticular anastomosis, which unites into two vessels the vasa omphalo-meseraica. (_British and Foreign Med. Rev._ No. 1.) These may be demonstrated with great ease in the chick: the cicatricula increases in extent; it becomes vascular, and at length forms a heart-shaped net-work of delicate vessels, which unite into two trunks, terminating one on each side of the abdomen.

_c_ The duct of the vesicula umbilicalis, dividing into two intestinal portions; and besides this duct are two vessels which are distributed upon the vesicula umbilicalis, and form a reticular anastomosis with each other. _From Baer._]

The umbilical vesicle now begins to separate itself more and more from the abdomen of the foetus, merely a duct of communication passing to that portion of it which forms the intestinal canal. The first rudiment of the cord will be found at this separation; its foetal extremity remains for a long time funnel-shaped, containing, besides a portion of intestine, the duct of the vesicula umbilicalis, the vasa omphalo-meseraica (the future vena portæ,) the umbilical vein from the collected venous radicles of the chorion, and the early trace of the umbilical arteries. These last-named vessels ramify on a delicate membranous sac of an elongated form which rises from the inferior or caudal extremity of the embryo, viz. the _allantois_; whether this is formed by a portion of the mucous layer of the germinal vesicle, in common with the other abdominal viscera, appears to be still uncertain: in birds this may be very easily demonstrated as a vascular vesicle, arising from the extremity of the intestinal canal; and in mammalia, connected with the bladder by means of a canal called _urachus_: from its sausage-like shape, it has received the name of _allantois_.

The existence of an allantois in the human embryo has been long inferred from the presence of a ligamentous cord extending from the fundus of the bladder to the umbilicus, like the urachus in animals. But from the extreme delicacy of the allantois, and from its function ceasing at a very early period, it had defied all research, until lately when it has been satisfactorily demonstrated in the human embryo by Baer and Rathke. It occupies the space between the chorion and amnion, and gives rise occasionally to a collection of fluid between these membranes, familiarly known by the name of the liquor amnii spurius, which, strictly speaking is the liquor allantoidis.

The function of the allantois is still in a great measure unknown. In animals it evidently acts as a species of receptaculum urinæ during the latter periods of gestation; but it is very doubtful if this be its use during the earlier periods. It does not seem directly connected with the process of nutrition, which at this time is proceeding so rapidly, first by means of the albuminous contents of the vitellus, or vesicula umbilicalis, and afterwards by the absorbing radicles of the chorion; but, from analogy with the structure of the lower classes of animals, it would appear that it is intended to produce certain changes in the rudimentary circulation of the embryo, similar to those which, at a later period of pregnancy, are effected by means of the placenta, and after birth by the lungs, constituting the great functions of respiration.

In many of the lower classes of animals, respiration (or at least the functions analogous to it) is performed by organs situated at the inferior or caudal extremity of the animal: thus for instance, certain insect tribes, as in hymenoptera, or insects with a sting, as wasps, bees, &c.; in diptera, or insects with two wings, as the common fly; and also the spider tribe, have their respiratory organs situated in the lower part of the abdomen. In some of the crustacea, as, for instance, the shrimp, the organs of respiration lie under the tail between the fins, and floating loosely in the water. Again, some of the molusca, viz. the cuttle-fish, have the respiratory organs in the abdomen. We also know that many animals, during the first periods of their lives, respire by a different set of organs to what they do in the adult state: the most familiar illustration of this is the frog, which, during its tadpole state, lives entirely in the water.

As the growth of the embryo advances, other organs whose function is as temporary as that of the allantois, make their appearance: these also correspond to the respiratory organs of a lower class of animals, although higher than those to which we have just alluded,--we mean bronchial processes or gills. It is to Professor Rathke (_Acta Naturæ Curios._ vol. xiv,) that we are indebted for pointing out the interesting fact, that several transverse slit-like apertures may be detected on each side the neck of the embryo, at a very early stage of development. In the chick, in which he first observed it, it takes place about the fourth day of incubation: at this period the neck is remarkably thick, and contains a cavity which communicates inferiorly with the oesophagus and stomach, and opens externally on each side by means of the above-mentioned apertures, precisely as is observed in fishes, more especially the shark tribe; these apertures are separated from each other by lobular septa, of exceedingly soft and delicate structure. Rathke observed the same structure in the embryo of the pig and other mammalia; and Baer has since shown it distinctly in the human embryo. It is curious to see how the vascular system corresponds to the grade of development then present: the heart is single, consisting of one auricle and one ventricle; the aorta gives off four delicate, but perfectly simple branches, two of which go to the right, and two to the left side; each of these little arteries passes to one of the lobules or septa at the side of the neck, which correspond to gills, and having again united with the three others, close to what is the first rudiment of the vertebral column, they form a single trunk which afterwards becomes the abdominal aorta. In a short time these slit-like openings begin to close; the bronchial processes or septa become obliterated, and indistinguishable from the adjacent parts; the heart loses the form of a single heart; a crescentic fold begins to mark the future division into two ventricles, and gradually extends until the septum between them is completed. It is also continued along the bulb of the aorta, dividing it into two trunks, the aorta proper and pulmonary artery; at the upper part the division is left incomplete, so that there is an opening from one vessel to the other, which forms the ductus arteriosus.[27] A similar process takes place in the auricles, the foramen ovale being apparently formed in the same manner as the ductus arteriosus; these changes commence in the human embryo about the fourth week, and are completed about the seventh.

At first the body of the embryo has a more elongated form than afterwards, and the part which is first developed is the trunk, at the upper extremity of which a small prominence less thick than the middle part, and separated from the rest of the body by an indentation, distinguishes the head. There are as yet no traces whatever of extremities, or of any other prominent parts; it is straight, or nearly so, the posterior surface slightly convex, the anterior slightly concave, and rests with its inferior extremity directly upon the membranes, or by means of an extremely short umbilical cord.

The head now increases considerably in proportion to the rest of the body, so much so, that at the beginning of the second month, it equals nearly half the size of the whole body: previous to, and after this period, it is usually smaller. The body of the embryo becomes considerably curved, both at its upper as well as its lower extremity, although the trunk itself still continues straight. The head joins the body at a right angle, so that the part of it which corresponds to the chin is fixed directly upon the upper part of the breast; nor can any traces of neck be discerned, until nearly the end of the second month.

The inferior extremity of the vertical column, which at first resembles the rudiment of a tail becomes shorter towards the middle of the third month, and takes a curviture forwards under the rectum, in the fifth week the extremities become visible, the upper usually somewhat sooner than the lower, in the form of small blunt prominences. The upper close under the head, the lower near the caudal extremity of the vertebral column. Both are turned somewhat outwards, on account of the size of the abdomen; the upper are usually directed somewhat downwards, the lower ones somewhat upwards.

_a_ Vesicula umbilicalis already passing into the ventricular and rectum intestine at _g_. _b_ Vena and arteria omphalo-meseraica. _c_ Allantois springing from the pelvis with the umbilical arteries. _d_ Embryo. _e_ Amnion. _f_ Chorion. _From Carus._]

The vesicula umbilicalis may still be distinguished in the second month as a small vesicle, not larger than a pea, near the insertion of the cord, at the navel, and external to the amnion. From the trunk, which is almost entirely occupied by the abdominal cavity, arises a short thick umbilical cord, in which some of the convolutions of the intestines may still be traced. Besides these it usually contains, as already observed, the two umbilical arteries and the umbilical vein, the urachus, the vasa omphalo-meseraica, or vein and artery of the vesicula umbilicalis, and perhaps, even at this period, the duct of communication between the intestinal canal and vesicula umbilicalis, the foetal extremity of which, according to Professor Oken's views, forms the processus vermiformis.

_a_ Chorion. _b_ The larger absorbent extremities, the site of the placenta. _c_ Allantois. _d_ Amnion. _e_ Urachus. _é_ Bladder. _f_ Vesicula umbilicalis. _g_ Communicating canal between the vesicula umbilicalis and intestine. _h_ Vena umbilicalis. _i i_ Arteriæ umbilicales. _l_ Vena omphalo-meseraica. _k_ Arteria omphalo-meseraica. _n_ Heart. _o_ Rudiment of superior extremity. _p_ Rudiment of lower extremity. _From Carus._]

The hands seem to be fixed to the shoulders without arms, and the feet to adhere to the ossa illi; the liver seems to fill the whole abdomen; the ossa innominata, the ribs, and scapulæ are cartilaginous.

In a short time the little stump-like prominences of the extremities become longer, and are now divided into two parts, the superior into the hand and the fore arm, the inferior into the foot and leg; in one or two weeks later, the arms and thighs are visible. These parts of the extremities which are formed later than the others, are at first smaller, but as they are gradually developed they become larger. When the limbs begin to separate into an upper and lower part, their extremities become rounder and broader, and divided into the fingers and toes, which at first are disproportionately thick, and until the end of the third month are connected by a membranous substance analogous to the webbed feet of water birds; this membrane gradually disappears, beginning at the extremities of the fingers and toes, and continuing the division up to their insertion. The external parts of generation, the nose, ears, and mouth appear after the development of the extremities. The insertion of the umbilical cord changes its situation to a certain degree; instead of being nearly at the inferior extremity of the foetus as at first, it is now situated higher up on the anterior surface of the abdomen. The comparative distance between the umbilicus and pubis continues to increase, not only to the full period of gestation, when it occupies the middle point of the length of the child's body, as pointed out by Chaussier, but even to the age of puberty, from the relative size of the liver becoming smaller.

Though the head appears large at first, and for a long time continues so, yet its contents are tardy in their development, and until the sixth month the parietes of the skull are in great measure membranous or cartilaginous. Ossification commences in the base of the cranium, and the bones under the scalp are those in which this process is last completed.

The contents of the scull are at first gelatinous, and no distinct traces of the natural structure of the brain can be identified until the close of the second month; even then it requires to have been sometimes previously immersed in alcohol to harden its texture. There are many parts of it not properly developed until the seventh month. In the medulla spinalis no fibres can be distinguished until the fourth month. The thalami nervorum opticorum, the corpora striata, and tubercula quadrigemina, are seen in the second month; in the third, the lateral and longitudinal sinuses can be traced, and contain blood. In the fifth we can distinguish the corpus callosum; but the cerebral mass has yet acquired very little solidity, for until the sixth month it is almost semi-fluid. (Campbell's _System of Midwifery_.)

About the end of the third, during the fourth, and the beginning of the fifth months, the mother begins to be sensible of the movements of the foetus. These motions are felt sooner or later, according to the bulk of the child, the size and shape of the pelvis, and the quantity of fluid contained in the amnion, the waters being in larger proportionate quantity the younger the foetus.

The secretion of bile, like that of the fat, seems to begin towards the middle of pregnancy, and tinges the meconium, a mucous secretion of the intestinal tube which had hitherto been colourless, of a yellow colour. Shortly after this the hair begins to grow, and the nails are formed about the sixth or seventh month. A very delicate membrane (membrana pupillaris,) by which the pupil has been hitherto closed, now ruptures, and the pupil becomes visible. The kidneys, which at first were composed of numerous glandular lobules (seventeen or eighteen in number,) now unite, and form a separate viscus on each side of the spine; sometimes they unite into one large mass, an intermediate portion extending across the spine, forming the horse-shoe kidney.

Lastly, the testes, which at first were placed on each of the lumbar vertebræ, near the origin of the spermatic vessels, now descend along the iliac vessels towards the inguinal rings, directed by a cellular cord, which Hunter has called _Gubernaculum testis_: they then pass through the openings carrying before them that portion of the peritoneum which is to form their tunica vaginalis.

The length of a full-grown foetus is generally about eighteen or nineteen inches; its weight between six and seven pounds. The different parts are well developed and rounded; the body is generally covered with the vernix caseosa;[28] the nails are horny, and project beyond the tips of the fingers, which is not the case with the toes; the head has attained its proper size and hardness; the ears have the firmness of cartilage; the scrotum is rugous, not peculiarly red, and usually containing the testes. In female children the nymphæ are generally covered entirely by the labia, the breasts project, and in both sexes frequently contain a milky fluid. As soon as a child is born, which has been carried the full time, it usually cries loudly, opens its eyes, and moves its arms and legs briskly; it soon passes urine and fæces, and greedily takes the nipple. (Naegelé's _Hebammenbuch_.)

Thus, then, in the space of forty weeks, or ten lunar months, from an inappreciable point, the foetus attains a medium length of about eighteen or nineteen inches, and a medium weight of between six and seven pounds. As these observations on the development of the ovum show that the structural arrangement of the embryo undergoes a succession of changes, by which it gradually rises from the lowest to the highest scale of formation, so we shall find it furnished with a succession of means for its nutrition, each corresponding more or less to the particular grade of development which it may have attained. Its earliest source of nourishment is doubtless the vitellus, or albuminous contents of the vesicula umbilicalis. The radicle or primitive trace, in this respect, bears a strong analogy to the seed of a plant; it brings with it its own supply of nourishment for its first stage of growth; in the latter, the cotyledons afford nourishment to the little plumula, until, by the formation of roots and absorption of moisture from the surrounding soil, it is enabled to support the early rudiment of the future plant. The early function of the chorion is very analogous to that of roots; it is an absorbing apparatus, collecting nourishment by means of its numerous absorbing fibrillæ: hence, according to Lobstein, the umbilical vein exists for some time previous to the umbilical arteries, and seems to perform an office in the foetus similar to that of the thoracic duct at a later period; its radicles or absorbing extremities seem to absorb a milky fluid, which after the first two months is found in the placenta, and which must be looked upon as a means of nourishment which does not exist in the latter months. This milky fluid was noticed by Leroux, who even then expressed his doubts, whether the radicles of the umbilical vein receive blood from the mother, or whether they only serve to absorb a white fluid which resembles chyle. In some manuscript notes of Dr. Young's lectures, which were taken by the late Dr. Parry, of Bath, when a student at Edinburgh, we find the following observation: "There is evidently in the placenta, besides blood-vessels, some other substance, which serves to absorb juices from the uterus, and to convert these into a chylous matter proper to nourish the foetus, and this matter is absorbed by the umbilical veins. This seems to be proved from the consideration of the placenta of animals which have cotyledons; for, on squeezing these glandular substances, we force out a sort of chylous liquor, and these are surrounded by the placenta, which absorb their liquor and convey it to the foetus."

The absorbing power of the umbilical vein continues till the fifth month; during the second or third, the foetus receives a good deal of nourishment from the liquor amnii, which at this period contains a considerable quantity of albuminous matter; this diminishes in the latter months of pregnancy. Moreover the body of the foetus begins to be covered with the vernix caseosa towards the seventh month, so that in the eighth and ninth months the absorption of liquor amnii by the skin is considerably impeded.

How far the full formed placenta, as seen after the fifth month, serves as a means of nutrition to the foetus, may still be a matter of doubt; its chief use after this period is, as we have already shown, for the purpose of producing certain changes in the blood of the foetus analogous to those of respiration;[29] still, however, it would seem that its function of nutrition is not entirely at an end, even at a late period of pregnancy. The numerous little granules of phosphate of lime, which are frequently found on the uterine surface of a full-grown placenta at a time when ossification is rapidly advancing in the foetal skeleton, would surely lead us to infer that the placenta in some way or other supplies the materials for this process.

_Foetal circulation._ We have already shown, that, in the early stages of development, the heart of the embryo is single, consisting of one auricle and one ventricle; that a septum gradually divides these into two parts until the double heart is formed, leaving two openings of communication between the right and left sides, the one between the auricles called the _foramen ovale_, the other between the pulmonary artery and aorta, viz. the _ductus arteriosus_.

From these and other peculiarities it will be seen that the foetal circulation differs essentially from that of a child after birth; and, in order to comprehend the nature and mechanism of the changes which take place in it when respiration first commences, it will be necessary that these peculiarities should be thoroughly understood. The condition of the foetus must also be remembered: surrounded by the liquor amnii, the foetus does not respire; its lungs have as yet been unemployed; they are therefore small and collapsed, and present a firm solid mass, nearly resembling liver in appearance. In this state but little blood from the pulmonary arteries can circulate through them; for, as the extreme ramifications of these vessels are distributed upon the mucous membrane lining the bronchi and air-cells, the free passage of blood through them will in great measure depend upon a previous condition of the air-cells. The pulmonary arteries in the foetal state are therefore small, and transmit but a small quantity of blood into their numerous ramifications, just sufficient to keep pervious these vessels which after birth are to be so greatly distended: in this state the lungs when thrown into water sink.

Hence, as the pulmonary arteries do not afford a sufficiently free exit to the contents of the right side of the foetal heart, nature has provided it with a peculiar means for carrying off the overplus quantity of blood, which is poured into the right auricle from the vena cava. This is attained first by the _foramen ovale_, an oval-shaped opening in the septum between the right and left auricles, and furnished with a semilunar valvular flap, so constructed, as to allow a free passage for the blood from the right to the left auricle, but none in the contrary direction. By this means a considerable quantity of blood is transmitted at once from the right to the left auricle, and, consequently, much less into the right ventricle and pulmonary artery. Still, however, more blood passes into the right ventricle than the pulmonary artery, in the collapsed state of the foetal lungs, is capable of conveying away. The pulmonary artery is therefore continued beyond its bifurcation into the aorta at its curvature, by means of the _ductus arteriosus_, which, in the full-grown foetus, forms a short thick passage between these two vessels; and in this manner is the right ventricle enabled to get rid of its surplus quantity of blood. Thus we see that the foetal heart although consisting of two auricles and two ventricles, continues to perform the functions only of a single heart, both ventricles assisting simultaneously to propel the same column of blood, viz. that of the aorta, and thus enabling the heart to act with considerable power.

The chief part of the blood, which flows through the iliac arteries, instead of being sent to the inferior extremities, is carried into the umbilical arteries, which passing up along the sides of the bladder meet the umbilical vein at the navel, and thus form the vessels of the umbilical cord. These arteries convey the blood of the foetus to the placenta, where, having undergone changes to which we have already alluded, it is returned by the umbilical vein. This vessel, which afterwards forms the round ligament of the liver, passes through the umbilicus along the anterior edge of the suspensory ligament; it supplies the left lobe with blood, and having given off a communicating branch to the vena portæ, which supplies the right lobe, it passes at once by a short passage, called _canalis venosus_, into the vena cava.

Thus, then, the peculiarities of the foetal circulation may be considered as four, viz. the _foramen ovale_, or passage from the right to the left auricle; the _ductus arteriosus_, or communication from the bifurcation of the pulmonary artery into the arch of the aorta; the _umbilical arteries_ arising from the iliac arteries, and carrying the blood along the cord into the placenta; and, lastly, the _canalis venosus_, or passage between the umbilical vein and vena cava.

Let us now examine the changes which take place in the foetal circulation at the moment of the child's birth. The child, which had hitherto been immersed in the bland and warm medium of the liquor amnii, is at once exposed to the action of the external air. By means of the sympathy existing between the skin and respiratory muscles, sudden and convulsive efforts at inspiration take place; the air-cells of the lungs become partially inflated, and, after a short time as the respiration increases in power and activity, become distended throughout their whole extent. The thorax rises; the flaccid diaphragm, which hitherto had been pushed up by the large foetal liver, now contracts, pressing down the liver into its natural situation. The lungs, from being a hard solid heavy substance, resembling liver, at once become inflated, elastic, and crepitous, light and permeable to air in every part.

The capillary terminations of the pulmonary artery, which ramify in the mucous membrane, forming the parietes of the air-cells, and which hitherto had been firmly compressed by the collapsed state of the foetal lungs, are suddenly rendered pervious throughout their whole extent. By this means, a vacuum, as it were, is formed in the ramifications of the pulmonary artery; each inspiration is accompanied by a rush of blood from the right ventricle into the newly-inflated structure. The pulmonary artery, at its bifurcation, swells and becomes turgid: the blood is carried off into its numerous ramifications as fast as the right ventricle can supply it; this may be easily understood from the law, in anatomy, viz. that the area of two arteries is greater than that of the trunk from which they bifurcate. From this state of distension, the distance between the pulmonary artery and the aorta is increased; the ductus arteriosus, which has now become empty, is stretched, and thus partially closed; the right auricle, which, but for the foramen ovale, could not have cleared itself of the whole quantity of blood which was poured into it from the vena cava, is now enabled to transmit its entire contents into the right ventricle; the left auricle, which before birth was supplied only by the foramen ovale from the right auricle, is now rapidly filled by the blood brought into it by the four pulmonary veins;--the equilibrium between the two auricles becomes altered;--the right, which hitherto had been somewhat gorged with blood, is now able to clear itself with facility; whereas, the left, which was but partially supplied, is now distended with a much greater quantity: there is now rather a disposition for the blood to regurgitate from the left to the right auricle; this, however, is prevented by the semilunar fold of the foramen ovale, which now acts as a valve, and generally becomes firmly attached to the septum. The obliteration of the canalis venosus at the posterior margin of the liver, and of the umbilical vein at the anterior edge, may, we think, be explained by the changes which necessarily follow the inflation of the lungs: the diaphragm, when it contracts, pulls down the liver into its natural situation; the distance, therefore, between the liver and the heart is increased, and the canalis venosus is consequently stretched, and considerably pressed upon, and precisely the same results follow with the umbilical vein.