182 RICHARD ASSHETON. 



The segmented ovum of the rabbit lies within a thick, tough 

 spherical coat, and is spherical. The ovum of the rat is not 

 thus placed within a spherical mould, and can therefore take 

 other forms, which from the conditions would seem to be 

 necessarily disc-like or oval. 



The blastodermic cavity (" Dottersackhohle " of Salenka, 

 " vitelline cavity " of Robinson) would seem to be produced 

 by a hydrostatic pressure from within, as in the rabbit, but 

 owing to there being no resistance of albumen layer the walls 

 of the cavity of the vesicle offer less resistance. Thus there is 

 less tension, and this has the effect of producing a shape of no 

 symmetry except such as is given to it by the walls of the 

 uterus in which it lies. 



In the rabbit an early tendency to rapid growth of the area 

 immediately surrounding the embryonic disc gives rise to an 

 expansion of that part of the vesicle; so, also, the same tendency 

 to a rapid growth occurs in the rat round the corresponding 

 area, but with very different results. A heap of cells accu- 

 mulates, which gives rise to the irregular mass growing 

 among the cavities of the uterus, and usually called Trager. 

 After a while the blastodermic vesicle becomes fixed in the 

 walls of the uterus, and the condition then is similar to 

 that in the rabbit, i.e. the necessary resistance is supplied, and 

 a more or less spherical swelling appears upon the uterus, in 

 which the embryo develops in safety. 



As regards the actual fixing of the rat embryo, this is no doubt 

 primarily due to the pressure exerted between the irregularities 

 of the walls of the uterus, the " trager " (and other external 

 layer cells of the embryo), this pressure being brought about by — 



(i) Multiplication of cells of trager and general growth of 

 embryo. 



(ii) Hydrostatic pressure within the blastodermic vesicle. 



Thus we see that what corresponds to the traj^er in the 

 rabbit becomes obvious in the rat at a very early period; 

 whereas in the rabbit its presence (essentially as an area of 

 increased energy) is marked by the continued expansion of 

 the vesicle, as described in a former paper. 



