54 THE FIKST DAY. [CHAP. 



Sections at this stage throw a very important light 

 on the nature and mode of origin of the primitive 

 streak. In the region in front of it the blastoderm is 



o 



still formed of two layers only, but in the region of the 

 streak itself the structure of the blastoderm is greatly 

 altered. The most important features in it are repre- 

 sented in Fig. 15. This figure shews that the median 



Fig. 15. 



Transverse Section throdgh a Blastoderm of about the 



AGE represented IN FiG. 14, SHEWING THE FlHST DIF- 

 FERENTIATION OP THE Primitive Streak. 



The section passes through about the middle of the primitive 

 streak. 



pvs. primitive streak ; ep. epiblast ; hy. hypoblast ; yk. yolk of 

 the germinal wall. 



portion of the blastoderm has become very much thick- 

 ened (thus producing the opacity of the primitive streak), 

 and that this thickening is caused by a proliferation of 

 rounded cells from the epiblast. In the very young 

 primitive streak, of which Fig. 15 is a section, the rounded 

 cells are still continuous throughout with the epiblast, but 

 they form nevertheless the rudiment of the greater part 

 of a sheet of mesoblast, which will soon arise in this 

 region. 



