AVES. 



125 



large extent by the difficulties of the investigation, but perhaps still more 

 by the fact that many of the observations were made at a time when the 

 methods of making sections were very inferior to those of the present day. 

 The subject itself is by no means of an importance commensurate with the 

 attention it has received. The chai*acters which belong to the formation of 

 the layers in the Sauropsida are secondarily derived from those in the Ich- 

 thyopsida, and are of but little importance for the general questions which 

 concern the nature and origin of the germinal layers. In the accoxmt in the 

 sequel I have avoided as much as possible discussion of controverted points. 

 My statements are founded in the main on my own observations, more espe- 

 cially on a recent investigation carried on in conjunction with my pupil, 

 Mr Deighton. It is to Kcilliker (No. 135), and to Gasser (Na 127) that 

 the most important of the more recent advances in our knowledge are due. 

 Kolliker, in his great work on Embryology, definitely established the 

 essential connection between the primitive streak and the formation of 

 the mesoblast ; but while confii'ming 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 trans- 

 parent region. The posterior opacity is named by some authors the 

 embryonic shield. 



T)uring 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 hypoblast spheres of the 

 un incubated blastoderm flatten themselves out, and unite into a 

 definite hypoblastic membrane (fig. 92). Between this membrane 



Fig. 92. Transverse section through the blastoderm of a chick before 



THE appearance OF THE PRIMITIVE STREAK. 



The epiblast is represented somewhat diasrammatically. The hyphens shew the 

 points of junction of the two halves of the section. 



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 



