SEGMENTATION OF THE OVUM OP THE SHEEP. 223 



In the bird's egg at the time of laying, and for the first few 

 hours of incubation, there is said to be a discontinuity between 

 the hypoblast and the ''yolk" (B). The yolk at this time is 

 nevertheless laden with nuclei (Duval, 23). After the first few 

 hours of incubation the yolk again becomes continuous with the 

 hypoblast, as in diagram C. 



The passage which is caused by this temporary loss of con- 

 tinuity, and which extends round the blastoderm, has been re- 

 garded as blastopore, and has been said to become converted 

 into the primitive groove. Some experiments made with a 

 view of testing this point by myself (4) proved the assumption 

 that it becomes converted into the primitive groove to be un- 

 tenable. 



It is, I think, reasonable to suppose that this loss of con- 

 nection between the two portions of hypoblast is in some way 

 related to the act of laying of the egg, and to the cessation of 

 development attendant on it. 



Fig. B, PL 18, can be compared with the second diagram 

 (p. 222) as regards the doubling of the hypoblast cells, amongst 

 which, in the next stage, the great cavities — the subgerminal 

 cavity, and the cavity of the blastodermic vesicle — subse- 

 quently appear. In most mammals, however, the hypoblast 

 temporarily extends over the epiblast, sothat the latter is com- 

 pletely withdrawn from the surface during a certain period of 

 development. 



The exact method by means of which this overgrowth in the 

 mammalian ovum is effected, and the method by which it is 

 subsequently rectified, so as to bring the epiblast again into 

 its proper position, seem to vary with almost every species. 

 The series of small diagrams on PI. 18 illustrate some of these 

 diverse methods, and their relations to each other. 



Fig. D, PI. 18, represents the segmenting ovum of possibly 

 all placental mammals. 



The pink represents epiblast, the green hypoblast. The 

 black ring is the zona radiata, and other investing coats which 

 may be present. 



Immediately below this is the next stage (X) of a certain 



