246 The Alligator and Its Allies 



Figure 6/z passes through the blastopore iblp). 

 The appearance of the section is almost identical 

 with that of Figure 2/, already described. 



Figure 6i is five sections posterior to the preced- 

 ing and has about the same structure as the corre- 

 sponding sections in the preceding two stages, where 

 this region of the embryo was injured, and hence 

 not drawn. Continuous with the posterior bor- 

 der of the blastopore (seen in the preceding fig- 

 ure) is the deep furrow, the primitive groove {pg) . 

 The ectoblast {ec) bordering this groove is much 

 thickened and may be called the primitive streak. 

 The lower side of this primitive streak is continuous 

 with the mesoblast {mes) , while the entoderm {en) 

 is here entirely distinct from the mesoderm. It 

 is evident that the mesoderm posterior to the 

 blastopore is proliferated from the lower side of the 

 ectoblast and not from the upper side of the ento- 

 blast, as is the case anterior to the blastopore. 

 The primitive groove gradually becomes more and 

 more shallow, as it is followed toward the poste- 

 rior, until it is no longer discernible ; back of this 

 point the primitive streak may be traced for a con- 

 siderable distance, becoming thinner and thinner 

 until it too disappears, and there remains only the 

 slightly thickened ectoblast underlaid by the thin 

 and irregular layers of mesoblast and entoblast. 

 The primitive streak may be traced for a distance 

 equal to about one third the distance between 

 the head-fold and the blastopore. 



