DEVELOPMENT OF OTHER VERTEBRATES 



429 



embryo develops and assumes the organization of the adult may be 

 compared with those occurring in the frog, if due allowance is made 

 for the modifying influence of the yolk. AVhen the neural plate is 

 developing into the neural tube (Fig. 224 D), the embryo is a 

 flattened mass of cells resting upon the yolk, as though in a frog 

 embryo, at a corresponding stage (Fig. 216), the yolk were so 

 increased that the embrj^o became a disc upon a sphere as large 

 as the yolk of the chick's egg. The flattened body of such a chick 

 embryo (Fig. 225) is covered by ectoderm. The endoderm is in 

 contact with the yolk laterally, while toward the center there is a 



Fig. 225. — Transverse section of chick embryo at a stage when the body plan 

 of the chordate is being estabhshed (c/. Fig. 216). 



a, aorta; c, notochord; cce, coelome; ec, ectoderm; en, endoderm; m, mesoderm; m..?., 

 muscle segment or mesoblastic somite; n, nerve cord; n.c, neural crest; n.t., neural tube; 

 V.V., vitelline vein; y, yolk. 



space, between the endoderm and the yolk, that may be called 

 an archenteron. The mesoderm is becoming differentiated into the 

 muscle segments, or mesoblastic somites, and is beginning its 

 separation into inner and outer layers to form the coelome. The 

 neural folds arise and fuse to form the neural tube, and the 7ioto- 

 chord is formed in a similar position in both frog and chick. In a 

 later stage these structures are further developed; but as long as 

 a considerable portion of the yolk remains, the lateral parts of the 

 embryo are spread widely as a thin plate. The margin of the 

 body, therefore, extends over the mass of yolk material, which is 

 gradually absorbed by the endoderm cells of the embryo and trans- 

 ferred by the peripheral blood vessels to the developing body. By 

 means of this abundant food supply, the embryo grows rapidly 

 and the yolk is correspondingly reduced (Fig. 224 F to G). 



