ATTACHMENT OF MAMMALIAN EMBRYO TO UTERUS. 183 



Then, again, let us look at the conditions of the mole. 

 Here is an animal, an insectivore, which is considerably re- 

 moved genetically from a rabbit. Yet here we find that 

 the development is extremely like that of the rabbit. It 

 is true that there is a slight approach to the inversion 

 condition, but only slight ; and I think that what appears 

 to be an inversion for a short time is very possibly in no 

 way connected with the inversion such as that of the house 

 mouse, field mouse, rat, or guinea-pig. The separation figured 

 by Heape (' Quart. Journ. Micr. Sci.,' 1883, PI. 29, figs. 24 

 and 25) seems to be due to quite other causes, namely, temporary 

 and rather sudden acceleration of growth of the embryonic 

 disc itself, and not of the area just beyond it. For the cells 

 which become separated from the epiblast, and which Heape 

 compares with the trager, afterwards again unite and fuse with 

 the epiblast and form together the epiblast of the embryonic 

 disc ; whereas in all other forms any cells which have once 

 become separated as "trager^' never participate in the formation 

 of the permanent epiblast. I should say these cells correspond 

 rather to the outer layer of epiblast in the embryonic disc in 

 the rabbit, and that in the mole the representatives of the 

 trager cells would be those which at this moment (Heape, 

 figs. 24 and 25) form part of the wall of the vesicle beyond the 

 region of the embryonic disc, as in the corresponding region 

 of the rabbit. 



Until the time of this temporary bending in of the embryonic 

 area, the development of the mole is exactly comparable to that 

 of the rabbit. The ovum segments and forms a spherical 

 morula. A blastodermic cavity and vesicle are formed almost 

 exactly comparable to the rabbit, and the hypoblast is formed 

 in the same way; the blastodermic vesicle is a spherical bladder- 

 like body ; and may not the reason of this be that the ovum 

 develops up to a certain point under quite similar conditions, 

 that is to say, surrounded by a thick protective covering of 

 zona pellucida and " mucous layer '^ (v. Heape) ? This mucous 

 layer develops rather differently, being applied according to 

 vHeape in the uterus, and not in the Fallopian tube. 



