EVOLUTION AS SEEN IN EMBRYONIC DEVELOPMENT 



53 



p\acen^o 



yolk soic 



oilloini"ois 

 hind gui 



FIG. 4.4. Human embryonic development during the third week. Longitudinal sec- 

 tions through the embryo and membranes. Only half of the embryo is shown. Ectoderm 

 indicated with solid black, endoderm with cross lines, mesoderm with fine dots. (After 

 Arey, Developmental Anatomy, W. B. Saunders Co., 1 947.) 



portion, the placenta, most directly connecting the embryo to the wall of 

 the uterus being shown. The embryonic disc becomes elongated, and the 

 cavities above and below it become enlarged. Almost immediately the 

 embryonic disc forms the beginning of the central nervous system. Two 

 parallel neural folds are thrust up into the overlying amniotic cavity 

 (Fig. 4.4 and Fig. 4.5A and B). If we think of the neural plate (that por- 

 tion of the embryonic disc which does this) as a flat plain, the neural folds 

 are like two parallel mountain ranges elevated above the surrounding 

 country. If, now, we can imagine such mountain ranges as growing higher 



