DEVELOPMENT OF THE PIG DURING FIRST TEN DAYS. 345 



possible that this internal layer might split off and give rise 

 to the embryonic hypoblast in the way Hubrecht describes for 

 Tupaia. 



As somewhat supporting this view I may draw attention to 

 the fact that the hypoblast is extremely attenuated, and very 

 reticular in the region of the embryonic area at this time. I 

 cannot positively assert that the hypoblast in many of my 

 specimens can for certain be seen lying against the region in 

 question. 



Also in the same sow there were two other specimens in 

 which the epiblast seemed to be reduced to little -more than a 

 single layer of large cells (or rather nuclei). The small nuclei 

 were, however, nowhere to be seen^ and the hypoblast was not 

 in any degree more evident. 



As militating against this view is the fact that in such speci- 

 mens as that drawn for fig. 28, the whole mass (E.) here is un- 

 doubtedly epiblast. This is proved by the presence of the thin 

 membrane {M.H.L.), as will appear in the description of the 

 older stages. In this case there is also a tendency to the for- 

 mation of an inner layer of smaller nuclei. This is probably 

 an older stage, its age being nine and a half days, but the 

 hypoblast here is by no means a continuous layer, such as it 

 is on the next day. A similar layer of small nuclei is also 

 plainly visible long after the hypoblast has become a well- 

 developed layer on the twelfth and thirteenth days. So, 

 although there are certain features which suggest a further 

 splitting off of the embryonic hypoblast comparable to what 

 Hubrecht has described for Tupaia, yet, from a consideration 

 of the evidence on both sides of the question, I am of the 

 opinion that this phenomenon does not occur in the pig, but 

 that the whole hypoblast is split off at the same time immedi- 

 ately after the rupture of the zona radiata ; and that the con- 

 dition shown in my figure is comparable to the condition shown 

 in Hubrecht's (17) figures 57 to 59, or ray figures of the sheep, 

 aud have nothing to do with the hypoblast formation. 



VOL. 41, PART 3. NEW SERIES. BB 



