122 Vertebrate Embryology 



and joining together, in a distinct membrane, 

 of these lower layer cells (Fig. 39). In 

 this process of the formation of the ento- 

 blast, some of the lower layer cells are left 

 between the ectoblast and the newly formed 



FIG. 40. SURFACE VIEW OF THE EMBRYO 

 AT ABOUT THE SIXTEENTH HOUR OF INCUBA- 

 TION. (After Duval.) 



m f, mesoblast forming the darker area in the pos- 

 terior region of the area pellucida. X, primitive streak. 



entoblast ; these scattered cells probably form, 

 as will be noted later, a part of the mesoblast 

 or middle germ layer. They are mostly con- 

 fined to the posterior end of the area pellu- 

 cida, where they form a slight opacity, which 

 is sometimes called the embryonic shield. 



