SEGMENTATION OF THE OVUM OP THE SHEEP. 219 



and physical causes, double over part of the wall in the majo- 

 rity of forms. 



The cavity of the blastocyst is not the segmentation cavity, 

 but the archenteron. It is bounded on all sides by hypoblast, 

 though at the part where it stretches across the epiblastic 

 disc it is perhaps a network of cells rather than a continuous 

 sheet at the time when the blastocyst cavity is very rapidly 

 expanding, as during the stages of figs. 12 — 16. 



There would seem to be nothing in the pig to cause a 

 differentiation in staining, but in the early stages there is a 

 distinct suggestion of a growth of smaller cells round a group 

 of larger ones. 



The bursting through of the epiblast and rejection of the 

 pieces of trophoblast torn apart in the process are particularly 

 clear in the pig. I regret very much that I have not got the 

 stages which may include the corresponding process in the 

 sheep (vide foot-note on p. 216). 



Concerning the homologies of the several parts of the 

 segmented ovum and the blastodermic vesicle of mammals 

 very diverse views have been held. 



It is only because the facts which I have discovered in the 

 early development of the sheep, which are corroborated by 

 events in the development of the pig, seem to give promise of 

 an interpretation in some ways more satisfactory than any 

 hitherto advanced, that I venture to enter upon a short discus- 

 sion of the subject. 



I am perfectly well aware that there is much to be said in 

 favour of the views expressed by embryologists far more com- 

 petent to deal with the subject than am I. As recently as 

 during the last two years important papers dealing with the 

 question of the segmentation of the mammalian ovum have 

 appeared from the pens of such noted embryologists as Pro- 

 fessors Hubrecht and Duval. Although the respective views of 

 these two authors were quite different, yet in each case most 

 plausible explanations were offered for many of the problems 

 of mammalian embryology. 



But to my mind neither of their views can be maintained 



