III.] THE HYPOBLAST. 51 



result in the formation of the three layers of the blasto- 

 derm. 



The upper layer, which is the epiblast already 

 spoken of (Fig. 1.3), takes at first but little share in 

 these changes. 



In the lower layer, however, certain of the cells 

 begin to get flattened horizontally, their granules become 

 less numerous, and the nucleus becomes distinct; the 

 cells so altered cohere together and form a membrane. 

 The membrane thus formed, which is first completed in 



FiQ. 13. 



Transverse Section through the Blastoderm of a Chick 



BEFORE THE APPEARANCE OF THE PRIMITIVE StRBAK. 



The epiblast is represented somewhat diagrammatically. The 

 hyphens shew the points of junction of the two halves of the 

 section. The hypoblast is already constituted as a membrane of 

 flattened cells, and a number of scattered cells are seen between 

 it and the epiblast. 



the centre of the pellucid area, constitutes the hypoblast. 

 Between the hypoblastic membrane and the epiblast 

 there remain a number of scattered cells (Fig. 13) 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 



4,-2 



