542 VERTEBRATE LIFE AND ORGANIZATION 



does not contribute to the neural tube iorms the epidermis, hair and 

 skin glands. 



262. The Digestive Tract and Its Derivatives 



The neural tube and embryo elongate laster than the embryonic 

 disc upon which the embryo is developing. As a result, the embryonic 

 disc buckles at each end. The embryo continues to elongate, and the 

 parts of the embryonic disc that originally lay anterior or posterior to 



Embryo- 



rAmnion 



-Forcgut 



Hindout 



Heart 



Body sta-lK 

 > ^--\ A llarAois 



Chorion 



H<z.art 



Embryo-i ^^ pAmnioxi. 



Hearb ■ 



'Allantois 



Live.r 

 primordium 



Mouth, pocket 

 (Stomodaeum) 



Umbiliccd cord- 



is al pcoicrea-S 



•ArchentcT-on. 



AnalpocKet 



(p r o ct o d.a.iz.ij.m.) 



AllantoiS 



Chorion 



Yolk 



sa.c 



c 



Figures 31.5. A series of diagrams of sagittal sections of embryos of different ages 

 to sliow the folding processes that separate the embryo from its extraembryonic mem- 

 branes. Sohd lines represent ectoderm, broken lines endoderm, and stippled lines and 

 shaded areas mesoderm. (Modified after Arey.) 



the neural tube fold underneath the embryo (Fig. 31.5). Folds first 

 separate the head and tail from surrounding structures. These folds 

 deepen and the folding process continues along each side until the em- 

 bryo is more or less cylindrical in shape and remains connected to its 

 surrounding membranes only by a narrow umbilical cord. The folding 

 process is somewhat analogous to the gradual tightening of a pair of 

 purse strings. 



These folding processes gradually pinch off the dorsal part of the 

 yolk sac and convert it into the primitive gut, or archenteron, of the 



