Chapter XVII: Tracheata (4)

The mesoblastic bands are very similar to, and probably homologous with, those of Chætopoda; but the different modes by which they arise in these two groups are very striking, and probably indicate that profound modifications have taken place in the early development of the Tracheata. In the Chætopoda the bands are from the first widely separated, and gradually approach each other ventrally, though without meeting. In the Tracheata they arise from the division of an unpaired ventral plate.

The further history of the mesoblastic bands is nearly the same for all the Tracheata so far investigated, and is also very much the same as for the Chætopoda. There is a division into somites, each containing a section of the body cavity. In the cephalic section of the mesoblastic bands a section of the body cavity is also formed. In Arachnida, Myriapoda, and probably also Insecta, the body cavity is primitively prolonged into the limbs.

In Spiders at any rate, and very probably in the other groups of the Tracheata, a large part of the mesoblast is not derived from the mesoblastic plate, but is secondarily added from the yolk cells.

In all Tracheata the yolk cells give rise to the mesenteron which, in opposition, as will hereafter appear, to the mesenteron of the Crustacea, forms the main section of the permanent alimentary tract.

One of the points which is still most obscure in connection with the embryology of the Tracheata is the origin of the embryonic membranes. Amongst Insects, with the exception of the Thysanura, such membranes are well developed. In the other groups definite membranes like those of Insects are never found, but in the Scorpion a cellular envelope appears to be formed round the embryo from the cells of the blastoderm, and more or less similar structures have been described in some Myriapods (_vide_ p. 390). These structures no doubt further require investigation, but may provisionally be regarded as homologous with the amnion and serous membrane of Insects. In the present state of our knowledge it does not seem easy to give any explanation of the origin of these membranes, but they may be in some way derived from an early ecdysis.