54 THE FIRST 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 

 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 THROUGH A BLASTODERM OF ABOUT THE 

 AGE REPRESENTED IN FlG. 14, SHEWING THE FlRST DIF- 

 FERENTIATION OF THE PRIMITIVE STREAK. 



The section passes through about the middle of the primitive 

 streak. 



pvs. primitive streak ; ep. epiblast ; Jiy. 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. 



