DEVELOPMENT OF THE EXTERNAL FORM OF THE BODY. 



141 



From a comparison of the three stages which have been mentioned, it can be 

 inferred that the process which establishes the cylindrical form of the body is 

 essentially one of bending of the margins of the embryonic disk with accom- 

 panying elongation of the disk. It is obvious that the process begins at an early 

 period — coincident with the appearance of the primitive streak and neural 

 groove. The margins of the disk bend ventrally and form the lateral body walls 

 (Figs, go, C, and 84) , then bend inward and finally meet in the midventral line 

 to form the ventral body wall. At the same time the body gradually be- 

 comes elongated in the direction of the neural groove (Fig. 119). When the 

 body walls bend inward a constriction is produced between the body and the 



Fore-brain 



1 fossa 



Heart 



Omphalomesenteric 

 vein 



_ Yolk sac 



" Amnion 



Belly stalk 



Fig. 120. — Dorso-lateral view of human embryo with fourteen pairs of primitive 

 segments (2.5 mm.). Kallmann. 



yolk sac. As the body and yolk sac enlarge, the constriction becomes relatively 

 deeper until the yolk sac is attached to the ventral side of the body by a slender 

 cord — the yolk stalk (Fig. 123). While in the earlier stages there is an active 

 bending of the margins of the disk, in the later stages the body grows rapidly in 

 size, especially in length, and extends out beyond the yolk sac (Fig. 120). This 

 makes it appear that the yolk stalk is becoming smaller. As a matter of fact, 

 the diminution in the relative size of the yolk stalk is more apparent than real, 

 the apparent diminution being caused largely by the rapid increase in size of the 

 embryonic body and yolk sac. There is, however, a considerable distance where 

 fusion occurs in the midventral line as the two lateral body walls meet to form 



