344 RICHARD ASSHETON. 



a continuous membraue {H, figs. 36 — 42). On the earlier 

 parts of the eighth and ninth days it is, however, a network 

 over all parts, and is still absent from the lower pole. Whether 

 this extension is effected by an active advance of its edge, or is 

 only an apparent advance due to the more active growth of the 

 circum-embryonic part of the trophoblast, as 1 believe to be the 

 case in the rabbit, I cannot say. During these two days the 

 blastocyst increases in size from about '2 mm. to about 1"5 mm. 

 The chief interest undoubtedly lies in the fate of the epi- 

 blastic portion of the inner mass and the part of the tropho- 

 blast which at first overlies it and is quite distinct from it. 



Fig. 27 may be taken to represent quite typically the 

 condition of things at the end of the eighth day. The tropho- 

 blast cells are cubical with clearly marked boundaries, both in 

 section and in surface view. There is no change observable for 

 some days in the nature of these cells beyond the limits of 

 the embryonic epiblast. 



In that part of the trophoblast which covers the epiblastic 

 portion of the inner mass the following changes occur con- 

 currently with certain changes in the epiblast. I will describe 

 the two together. 



The diameter of the epiblast — that is to say, of the embryonal 

 area — as seen in surface view increases from about "04 mm. to 

 •25 mm. As it grows it becomes more firmly pressed against 

 the trophoblast, which at this spot becomes distinctly thinner 

 and more compressed, and its nuclei become much flattened 

 (fig. 28). 



The epiblast begins to show signs of more active growth, 

 and the cells which at first were loosely arranged and equal in 

 size, and showed no sign of differentiation, may now, in some 

 cases, be seen to be arranged in an inner layer of small nuclei 

 lying next to the cavity of the blastodermic vesicle, and a layer 

 of very much larger nuclei between this and the trophoblast 

 layer (fig. 29). 



This inner layer of smaller nuclei, which also stain rather 

 more deeply, is so very distinct in one of my specimens which 

 I obtained from a sow killed at eight days, tliat I thought it 



