128 RICHARD ASSHETON. 



eighth hour after coition. There is now less unanimity of action 

 in point of time of the division of the several cells, so that it 

 is fairly common to find embryos with five or seven segments. 

 Here, again, four cells may be distinctly smaller than the 

 remaining four, or all may be almost exactly equal in size. 

 There is no difference to be detected in the character of the 

 contents of the spheres. 



4. The segments continue to divide with less and less regu- 

 larity, so that the descendants of one primary segment become 

 mingled with those of the other, there being much difference 

 in size between the several segments. 



5. The result of this continued activity is the formation of 

 a solid " morula " of cells of varying size but similar 

 character, and it is impossible to apply the term epiblast or 

 the term hypoblast to any part of the embryo as yet. 



The ovum when it leaves the ovary is enclosed within a 

 sheath or protective investment, the zona radiata. As the 

 ovum proceeds down the oviduct this protective investment is 

 further strengthened by the deposition on its outer surface of 

 a thick coat of some albuminous substance which is secreted 

 by certain cells of the epithelial lining of the oviduct, and 

 which forms a tough strong membrane which has an important 

 influence on the future mode of development. 



CHAPTER II. 



The Formation of the Blastodermic Vesicle. 



The Fourth Day (73rd to 96th Hours). 



The most noticeable feature of this day's events is the 

 commencement of a cavity within the morula, which cavity 

 enlarges enormously. 



What the object of this cavity is we can pretty safely infer, 

 as also we can pretty safely conclude that the causes which 

 bring it about originated in the organism in connection with 

 the diminishing size of the ovum of the distant ancestral 

 animal ; but how this cavity is actually produced in the 



