DEVELOPMENT. 



135 



undergo their early development in the stomach of the mother. 

 Care of the brood is found most frequently in forms inhabiting 

 the colder seas. 



With a few exceptions the early development, so far as it is 

 at present known, may be summarized as follows. The egg is 

 fertilized in the sea, undergoes a total cleavage, and becomes 

 transformed into a hollow one-layered blastosphere. At one 

 pole of the blastosphere an in vagination makes its appearance 

 and a typical gastrula is formed (Fig. 87). The invaginated 

 cells constitute the endoderm, the cavity bounded by them the 

 archenteron, and the opening of the archenteron the blastopore. 

 The archenteron 

 rarely if ever fills 

 the segmentation 

 cavity, but the lat- 

 ter soon becomes 

 traversed by a 

 nucleated proto- 

 plasmic network 

 (Fig. 88), which is 

 continuous both 

 with the endoderm 

 and with the ecto- 

 derm (outer cells of 

 the gastrula), and 

 constitutes the first 

 trace of the meso- 

 derm. This meso- 

 dermal network, which is reinforced from the later appearing 

 enterocoelic vesicles is commonly called mesenchyme and gives 

 rise to the connective tissues, to some of the muscles, and to 

 the calcareous structures of the body. 



Different views may be held as to the origin and- structure of this early 

 appearing mesodermal network. The usual view is that it consists of 

 amoeboid cells which arise by the proliferation of the epithelial cells of 

 the embryo in the blastosphere and later stages, and wander into the 

 hlastocoel. Another view for which there is much to be said is that the 

 blastocoel at its very first appearance is traversed by protoplasmic pro- 

 cesses of its walls,* and that the apparent proliferation of cells of the 



\ 



FIG. 88. Gastrula stage of Tox&pneustes bremgpinosus (from 

 Korschelt and Heider, after Selenka). The mesodermal net- 

 work (so-called mesenchyme) is shown traversing the blasto- 

 coel and continuous with both ectoderm and endoderm. 



* See C. Shearer on the connexions between the blastomeres of embryos 

 in the Proc. Roy. Soc., 1906. 



