I 5 6 



FORMATION OF MESOBLAST. 



rounded cells of the mesoblastic involution of the primitive 

 streak. 



In the region in front of the primitive streak, where the first 

 trace of the embryo will shortly appear, the layers at first undergo 

 no important changes, except that the hypoblast becomes some- 

 what thicker. Soon, however, as shewn in longitudinal section 

 in fig. 98, the hypoblast along the axial line becomes continuous 

 behind with the front end of the primitive streak. Thus at this 



FIG. 98. LONGITUDINAL SECTION THROUGH THE AXIAL LINE OF THE 

 PRIMITIVE STREAK, AND THE PART OF THE BLASTODERM IN FRONT OF IT, OF 

 AN EMBRYO CHICK SOMEWHAT YOUNGER THAN FIG. 99. 



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

 streak ; . nuclei ; yk. yolk of germinal wall. 



point, which is the future hind end of the embryo, the mesoblast, 

 the epiblast, and the hypoblast all unite together; just as they 

 do in all the types of Ichthyopsida. 



Shortly afterwards, at a slightly later stage than that repre- 

 sented in fig. 96, an important change takes place in the constitu- 

 tion of the hypoblast in front of the primitive streak. The 

 rounded cells, of which it is at first composed (fig. 98), break up 

 into (i) a layer formed of a single row of more or less flattened 

 elements below the hypoblast and (2) into a layer formed of 

 several rows of stellate elements, between the hypoblast and the 

 epiblast the mesoblast (fig. 99). 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 



