348 EICHARD ASSHETON. 



trophoblast is lying over the surface of the epiblast, and is 

 broken. The age of these three specimens was nine days 

 eighteen hours. 



I have killed several animals in attempting to acquire a 

 more perfect series of this stage, but without success. There 

 is no possibility of obtaiuing with certainty embryos of a 

 given age. 



If we take the specimens of figs. 30 — 33 as representing the 

 ordinary course of development^ we see there is evidence of a 

 doubling up of, at any rate, a certain part of the epiblastic 

 plate ; but I do not think that it ever amounts to such a com- 

 plete folding as it does in Tupaia or Talpa. In any case it 

 is clear that considerable pieces of the ruptured trophoblast 

 may be left upon and more or less attached to the epiblastic 

 knob (v. figs. 31 — 35), which give rise to the conditions 

 described below, and which, I think, account for the appear- 

 ances described by Weysse. 



There is no regularity in the shapes of the pieces thus sepa- 

 rated, some more and some less completely, from each other, 

 and from the edge of the trophoblast adjoining the embryonic 

 area. 



In a certain specimen (fig. 10), the age of which was ten 

 days two hours, the trophoblast at one spot is seen in a section 

 to be overlying the embryonic epiblast (fig. 37, T.R.). The tro- 

 phoblast stains more darkly than the embryonic epiblast. On 

 following this overlapping edge through the series of sections, 

 it is found to run towards the centre of the embryonal area as 

 a narrow strip quite free from and rising away from it as it 

 advances towards the centre. At the centre it expands into a 

 fan-shaped plate, from the floor of which a pillar runs to the 

 embryonal area below, and is clearly fused with it at this spot 

 (fig. 36, T./?.). 



It is evident that here is a structure even more deserving of 

 the name of " bridge " than the lateral expansions of Weysse, 

 which may be likened rather to balconies. 



Over another part of the same embryonal area are a few 

 cells which lie more loosely upon the epiblast. 



