GENERAL ZOOLOGY 



Gill slit 



Eye 



Ear 



Neural fold 



Nostril Heart Pronephros Blastoderm 



Fig. 5.20. Embryo of the torpedo, an elasmobranch, attached to its yolk 

 sac. (Redrawn from H. E. Ziegler, Lehrbuch der vergleichenden Entwick- 

 lungsgeschtckte, 1902.) 



the excretory and reproductive systems are differentiated (Fig. S.ITQ. The 

 remaining mesoderm is the lateral mesoderm, or hypomere, the cells of which 

 become rearranged into an outer layer lying close to the ectoderm and an 

 inner layer lying next to the endoderm (Fig. 5.17.6 and C). The cavity of the 

 hypomere lying between these two layers is the coelom (Fig. 5.195). The 

 heart and main vessels of the circulatory system are established in the mesoderm 

 (Figs. 5.19Cand 5.215). 



Blastoderm 



Optic 



vesicle 



Metencephalon 

 Myelencephalon 

 Auditory pit 

 Gill slits 



Vitelline vein 



Neural 

 folds 



Mesencephalon 



Lens vesicle 

 Diencephalon 

 Optic cup 

 Telencephalon 

 Olfactory pit 

 Atrium ) 

 Ventricle ^ 



L CO 



-Sinus \^ 

 venosus J 



Vitelline 

 vein 



' i4« ^ vf-~.^Vitelline 

 1 11*0 ' ., artery 



Tail bud - 



B 



Fig. 5.21. Chick embryos, showing localization of organ-system primordia; dorsal view. A, 

 embryo after 29 hours of incubation. B, embryo after 52 hours of incubation, with its em- 

 bryonic membranes removed. Internal structures are shown as if seen through the surface. 

 (Redrawn from M. Duval, Alias d'embryologie, 1889.) 



156 



