222 



THE PRIMITIVE STREAK. 



commences the layer of hypoblastic mesoblast is present, espe- 

 cially just in front of, and at the sides of, the anterior part of the 

 streak; but no mesoblast is found in the anterior part of the 

 embryonic area. These features are shewn in fig. 138 A and B. 



A. 



FlG. 138. TWO SECTIONS THROUGH OVAL BLASTODERMS OF A RABBIT ON 

 THE SEVENTH DAY. THE LENGTH OF THE AREA WAS ABOUT I'2 MM. AND ITS 

 BREADTH ABOUT '86 MM. 



A. Through the region of the blastoderm in front of the primitive streak ; B. 

 through the front part of the primitive streak ; ep. epiblast ; m, mesoblast ; hy. hypo- 

 blast ; pr, primitive streak. 



The mesoblast derived from the proliferation of the epiblast soon 

 joins the mesoblast already present; though in many sections it 



FlG. 139. TWO TRANSVERSE SECTIONS THROUGH THE EMBRYONIC AREA OF AN 

 EMBRYO RABBIT OF SEVEN DAYS. 



The embryo has nearly the structure represented in fig. 140. 



A. is taken through the anterior part of the embryonic area. It represents about 

 half the breadth of the area, and there is no trace of a medullary groove or of the 

 mesoblast. 



B. Is taken through the posterior part of the primitive streak. 



ep. epiblast; hy. hypoblast. 



