AVES. 



in his great work on Embryology, definitely established the essential con- 

 nection between the primitive streak and the formation of the mesoblast ; 

 but while confirming his statement on this head, I am obliged to differ from 

 him with reference to some other points. 



Gasser's work, especially that part of it which relates to the passages 

 leading from the neural to the alimentary canal, which he was the first to 

 discover, is very valuable. 



The blastoderm gradually grows in size, and extends itself 

 over the yolk ; the growth over the yolk being very largely 

 effected by an increase in the size of the area opaca, which 

 during this process becomes more distinctly marked off from the 

 area pellucida. The area pellucida gradually assumes an oval 

 form, and at the same time becomes divided into a posterior 

 opaque region and an anterior transparent region. The posterior 

 opacity is named by some authors the embryonic shield. 



FIG. 92. TRANSVERSE SECTION THROUGH THE BLASTODERM OF A CHICK 



BEFORE THE APPEARANCE OF THE PRIMITIVE STREAK. 



The epiblast is represented somewhat diagrammatically. The hyphens shew the 

 points of junction of the two halves of the section. 



During these changes the epiblast (fig. 92) becomes two 

 layers deep over the greater part of the area pellucida, though 

 still only one cell deep in the area opaca. The irregular hypo- 

 blast spheres of the unincubated blastoderm flatten themselves 

 out, and unite into a definite hypoblastic membrane (fig. 92). 

 Between this membrane and the epiblast there remain a number 

 of scattered cells (fig. 92) which cannot however be said to form 

 a definite layer altogether distinct from the hypoblast. They are 

 almost entirely confined to the posterior part of the area 

 pellucida, and give rise to the opacity of that part. 



At the edge of the area pellucida the hypoblast becomes con- 

 tinuous with a thickened rim of material, underlying the epiblast, 

 and derived from the original thickened edge of the blastoderm 

 and the subjacent yolk. It is mainly formed of yolk granules, 



