226 RICHARD ASSHETON. 



remainder from Duval's (26) account of the bat. As, however, 

 Duval's explanation is quite different from the above, I have 

 given below at some length my reasons for dissenting from 

 his views. 



Erinaceus on this hypothesis would be in some respects 

 similar. 



Unfortunately the earliest stages of the hedgehog, with 

 the exception of a two-segment stage described by Keibel (37), 

 are not known. Fig. E is therefore quite hypothetical for 

 the hedgehog, and should probably be drawn without a zona, 

 and possibly at this stage the hypoblast should be shown 

 already extended over the epiblast. 



If the increase of the hypoblast cells is very much greater 

 proportionally to the epiblast than it is in the sheep, it is quite 

 possible that when the cavity of the blastocyst arises its walls 

 may be more than one layer thick, some of the inner cells 

 giving rise to the inner sac described by Hubrecht (33). 



In Hubrecht's earliest figures there is certainly no distinc- 

 tion between the trophoblast and epiblast, but in the slightly 

 older ones which are to be compared with my diagram E the 

 distinction is clear, as a glance at Hubrecht's figs. 39, 8, 8 a 

 will show. Figure E may be compared with fig. 15 or 16. 



I have drawn Figure Eg according to Hubrecht's figs. 20 h 

 and 51, although I am not clear whether he describes the forma- 

 tion of the true amnion as the advance of a free edge, as would 

 appear from fig. 51, or whether, as the wording of the text 

 seems to indicate, (see pp. 289 — 296, and again 374, where 

 he says, " We have seen that after the completion of the 

 germinal area its epiblast remains attached to the trophoblast 

 [from which it has split off] along a circular line of insertion. 

 At the time of the formation of the amnion this line of attach- 

 ment travels upwards, the circle becoming smaller and smaller, 

 until at the completion of the amnion no connection between 

 embryonic epiblast and trophoblast any longer exists," it is 

 really a process of splitting off from the overlying trophoblast). 



If, therefore, it is really a case of splitting, the amnion cavity 

 shown in Figures Eo and Eg should have been drawn within 



