488 



DEVELOPMENT OF PRIMITIVE BODY FORM 



in elasmobranch fishes, reptiles, birds, and mammals. This process in- 

 volves the formation of lateral body folds which fold mesially toward 

 the median plane. (Study fig. 241 A-D.) In teleost fishes the process 

 is different, for in this group the entoderm and mesoderm grow out- 

 ward beneath the primitive epidermis (ectoderm) and soon envelop 

 the yolk. Thus, the end result in teleosts is much the same as in the 

 frog and Nectiirus. It is well to observe, at this point, that a corhplete 

 retraction of the ventro-lateral walls of the midgut and body-wall 

 tissues surrounding the yolk or yolk-sac area, as in the frog and 

 Necturus (fig. 227), does not occur in the higher vertebrates, although 

 in the elasmobranch and teleost fishes such retraction does occur. 



TAIL REGION 



Fig. 238. Chick embryo of about 27 to 28 pairs of somites. (Corresponding approxi- 

 mately to Hamburger and Hamilton, '51, stage 16, 51 to 56 hours of incubation.) Fore- 

 brain (prosencephalon) is divided into telencephalon and diencephalon; epiphysis is ap- 

 pearing on roof of diencephalon; cephalic and cervical flexures are pronounced; tail bud 

 is short; anterior part of body is rotated to the left back to about the thirteenth pair 

 of somites; amnion now covers anterior three fifths of body; heart shows strong ven- 

 tricular loop; three pairs of aortal arches can be seen. (A) External view. (B) Trans- 

 parent wholemount." (C) Sagittal section, diagrammatic. 



