186 ETOHARD ASSHETON. 



bryo to the walls of the uterus. What actually brings about 

 the fixation in the embryo in Ungulates I do not know. It 

 would seem possible, from the frequency of a cotyledonary ' 

 placentation, that a number of spots of pressure arise at 

 considerable intervals amongst the folds of the uterus, and so 

 bring about local areas where the rate of growth of the cellular 

 wall is in excess of the rate of expansion of the elongated 

 vesicle. 



In connection with the elongation of the blastodermic 

 vesicle, I may draw attention to a fact in the development of 

 the rabbit. 



It follows from what I have said, that if the albumen-layer 

 in the rabbit was less thick, or absent, its development would 

 be very different. So it happens that after the rupture of the 

 albumen-layer there is a tendency for the vesicle to elongate 

 during the eighth and ninth days. The normal shape of the 

 vesicle upon the ninth day is shown in fig. 3. 



It is important for the development of the space (C. BL.) 

 that after the rupture of the albumen layer the passages {UT.) 

 should be closed ; otherwise, unless the thin wall of the blasto- 

 dermic vesicle is very much stronger at that point than 

 hitherto, it could not withstand the pressure from within, which 

 must now be considerable, and would rupture, and the liquid 

 would escape and the swelling (C. BL.) would collapse. In 

 the rabbit, no doubt, the mucous membrane of the approxi- 

 mated walls of the uterus at this point {UT.) is thrown into 

 folds and pressed together by becoming a little bent inwards, 

 and by this means tends to block the passage, but also, no 

 doubt, the horns of the vesicle [H.) also help to obliterate and 

 plug up the cavity at these points. 



Not unfrequently it occurs that either one of the horns (or 

 both) extends through this narrow passage {UT.) and is pro- 

 longed (the albumen layer having ruptured by now) for a short 

 distance along the cavity of the uterus. Sometimes a horn 

 seems to be prolonged for a distance of as much as 34 mm., but 

 the greater part of this is non-cellular (Z.), the origin of which 

 I have not traced, 



