320 THE MAMMALIAN EMBRYO. [CHAP. 



the primitive streak makes its appearance ; it is due to 

 a proliferation of rounded cells from the epiblast. 

 FIG. 101. 



p.r 



SECTION THROUGH AN OVAL BLASTODERM or A RABBIT ON 

 THE SEVENTH DAY. THE LENGTH OF THE AREA WAS 

 ABOUT 1'2 MM. AND ITS BREADTH ABOUT '86 MM. 



Through the front part of the primitive streak ; ep. epiblast ; 

 m. mesoblast ; hy. hypoblast ; pr. primitive streak. 



These cells give rise to a part of the mesoblastic 

 layer of the embryo, and may be termed from their 

 origin the primitive streak mesoblast. 



During the seventh day the primitive streak be- 

 comes a more pronounced structure (Fig. 101), the 

 mesoblast in its neighbourhood increases in quantity, 

 while an axial groove (Fig. 100) the primitive groove 

 is formed on its upper surface. 



The formation of the medullary groove. 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. 103). 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 remaining at its hinder end. While the 



