MAMAIALIA. 



185 



on its upper surface. The mesoblastic layer in front of the primitive 

 streak becomes thicker, and, in the two-layered region in front, the 

 epiblast becomes several rows deep (fig. 139 A). 



In the part of the embryonic area in front of the primitive streak 

 there arise during the eighth day two folds bounding a shallow 

 median groove, which meet in front, but 

 diverge behind, and enclose between them 

 the foremost end of the primitive streak 

 (fig. 141). These folds are the medullary 

 folds and they constitute the first definite 

 traces of the embryo. The medullary 

 plate bounded by them rapidly grows in 

 length, the primitive streak always re- 

 mainincf at its hinder end. While the 

 lateral epiblast is formed of several rows 

 of cells, that of the medullary plate is at 

 first formed of but a single row (fig. 142 mg). 

 The mesoblast, which appears to grow for- 

 ward from the primitive streak, is stated 

 to be at first a continuous sheet between 

 the epiblast and hypoblast (Hensen). The 

 evidence on this point does not however 

 appear to me to be quite conclusive. In any case, as soon as ever 



Fig. 140. Embeyonic area 



OF AN EIGHT DAYS' EaBBIT. 



(After Kolliker.) 



ar(j. embryonic area; pr. 

 primitive streak. 



Fig. 141. Embryonic area of a seven days' embryo Babbit. (From Kolliker.) 

 0. place of future area vasculosa; rf. medullary groove; pr. primitive streak; 

 ag. embryonic area. 



