PRIMITIVE STREAK TO SOMITE FORMATION 37 



be seen of a local differentiation of that region of the primitive 

 gut which underlies the anterior part of the embryo. By focus- 

 ing through the ectoderm in the anterior region of a whole- 

 mount of this age a pocket of entoderm can be seen (Fig. 14). 

 This entodermal pocket is the first part of the gut to acquire a 

 floor, other than the yolk floor, and is called from its anterior 

 position the fore-gut. Consideration of the fore-gut except to 

 note the location of its first appearance can advantageously be 

 deferred because its origin and relationships are more readily 

 appreciated from the study of somewhat older embryos. 



The Growth and Differentiation of the Mesoderm. The 

 mesoderm which arises from either side of the primitive streak 

 spreads rapidly laterad and at the same time each lateral 

 wing of the mesoderm swings cephalad. Figure 12 shows 

 schematically the extension of the mesoderm during the latter 

 part of the first day of incubation. The diagonal hatch- 

 ing represents the mesoderm seen through the transparent 

 ectoderm. The principal landmarks of the embryos are 

 sketchily represented. 



It will be noticed that the manner in which the mesoderm 

 spreads out leaves a mesoderm-free area in the anterior portion 

 of the blastoderm. This region is known as the proamnion. 

 The name might carry the inference that this area is the primor- 

 dium of the amnion, a structure which first appears near this 

 region somewhat later in development. Such is not the fact. 

 The term proamnion was applied to this region before its true 

 significance was understood. It is not the precourser of the 

 amnion. In dorsal views of entire embryos the proamnion is 

 readily located by reason of its lesser density. The proamnion 

 is bounded anteriorly by the area opaca, posteriorly in the mid- 

 line by the thickened anterior part of the embryo, and poste- 

 riorly on either side by the anterior bordero f the mesoderm 

 (Fig. 12). The importance of the proamnion lies chiefly in the 

 indication it gives of the progress of mesoderm extension. The 

 rapid growth that the mesoderm of the anterior region is under- 

 going at this stage is clearly indicated by the diminution in 

 area of the proamnion in embryos of 22 hours as compared with 

 embryos of 18 hours (Fig. 12). 



Sections passing through the primitive streak of embryos of 

 this stage show the pair of loosely aggregated masses of meso- 



