III.J FORMATION OF THE EMBETO. 59 



Fio. 17. 



Transvebs'e Section through the Feont End of the Pri- 

 mitive Streak of a Blastoderm of the same age as 

 Fie. 16. 



pe. primitive groove; m. mesoblast; ep. epiblast ; Ay. hypo- 

 blast ; yh. yolk of germinal wall. 



FiQ. 18. 



Longitudinal Section through the Axial Line of the 

 Primitive Streak, and the Part of the Blastoderm 

 IN Front of it, of the Blastoderm of a Chick some- 

 what YOUNGER than FiG. 19. 



pr.s. primitive streak ; ep. epiblast ; hy. hypoblast of region in 

 front of primitive streak ; n. nuclei ; yk. yolk of germinal 

 wall. 



embryo, the mesoblast, the epiblast, and the hypoblast 

 all unite together. 



From the 16th to the 20th hours. At about the 

 16th hour, in blastoderms of the stage represented in 

 Fig. 16, an important change takes place in the constitution 

 of the primitive hypoblast in front of the primitive streak. 

 The rounded cells, of which it is at first composed (Fig. 

 18), break up into (1) a layer formed of a single row of 

 more or less flattened elements below — the hypoblast 

 proper — and (2) into a layer formed of several rows of 

 stellate elements, between the hypoblast and the epiblast 

 — the mesoblast (Fig. 19m). A separation between these 

 two layers is at first hardly apparent, and before it has 

 become at all well marked, especially in the median line, 

 an axial opaque line makes its appearance in surface 

 views, continued forwards from the front end of the 

 primitive streak, but stopping short at a semicircular 



