54 INTRODUCTION TO EVOLUTION 



and higher, and then bending toward each other until their summits touch 

 and fuse together, we have a metaphor for the process by which the 

 neural folds form into a neural tube. As shown in Fig. 4.5C, fusion of the 

 folds occurs first in the middle of the "back" of the embryo, progressing 

 from that point toward the head or anterior end and toward the tail or 

 posterior end (Fig. 4.5D) as though being closed by zippers. The neural 

 tube is wider at the anterior end than it is in other regions of the body; 

 this anterior portion will form the brain, the more posterior portions the 

 spinal cord. 



neural plate 



primitive 

 streoik 



FIG. 4.5. Human embryonic development during the third and fourth weeks. Dorsal 

 view, showing the "back" of the embryo. (After Arey, Developmental Anatomy, W. B. 

 Saunders Co., 1947.) 



From Fig. 4.4C, it will be noted that while the beginnings of the central 

 nervous system are being thrust up into the amniotic cavity the under sur- 

 face of the embryo is also changing. The upper part of the cavity which 

 we have referred to as the yolk sac becomes partially separated from the 

 lower portion. The upper part is the beginning of the digestive tract and 

 corresponds to the archenteron of a typical gastrula; the anterior portion 

 of the tract is the foregut, the posterior portion the hind gut (Fig. 4.4C). 



The Human Yolk Sac 



The lower portion of the cavity beneath the embryonic disc remains as 

 the yolk sac. This is an appropriate time to inquire into its significance. 

 Since there is no yolk to be contained, how do we happen to have it? 



