224 



THE BLASTOPORE. 



servers. In doing so I shall leave out of consideration the fact 

 (fully dealt with later in this chapter) that the layers in the 

 guinea-pig are inverted. Fig. 143 represents a series of sections 

 through this part in the guinea-pig. The anterior section (D) 



FIG. 141. EMBRYONIC AREA OF A SEVEN DAYS' EMBRYO RABBIT. (From 

 Kolliker.) 



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

 ag. embryonic area. 



FIG. 142. TRANSVERSE SECTION THROUGH AN EMBRYO RABBIT OF EIGHT DAYS. 

 ep. epiblast ; me. mesoblast ; hy. hypoblast ; mg. medullary groove. 



passes through the medullary groove near its hinder end. The 

 commencement of the primitive streak is marked by a slight 

 prominence on the floor of the medullary groove between the 

 two diverging medullary folds (fig. 143 C, ae). Where this pro- 

 minence becomes first apparent the epiblast and hypoblast are 

 united together. The mesoblast plates at the two sides remain 



