188 RICHAED ASSHETON. 



the epithelium. Instead of this, the epiblast of the embryo 

 becomes very thick and amounts to almost a loose proliferation 

 of cells, which cells become eventually pushed into the 

 irregularities and glands of the placental lobes. 



Section 2. 



(i) The primary cause of "inversion" is the fact that the 

 embryonic area is at one time a region of less activity, and is 

 surrounded by a zone of greater activity in connection with the 

 future formation of placenta and a space within which the 

 embryo can develop unaffected by pressure external to itself. 



(ii) The occurrence of "inversion" is determined by the 

 production of a heaping-up of cells at an early stage by this 

 zone of greater activity and so forcing the embryonic area 

 inwards. 



(iii) Inversion is prevented by causes which impede the 

 heaping-up of tissues around the embryonic area. 



(iv) Foremost among these preventing causes is the presence 

 of an investing coat, either zona radiata^ or albumen layer, or 

 mucous coat, &c., which — 



(1) By hindering a close connection between the cells of 

 the blastodermic vesicle and the uterine walls ; 



(2) By affording a strong support to the walls of the blasto- 

 dermic vesicle — 



allow the blastodermic vesicle to assume, under the expanding 

 influence of the hydrostatic pressure within, shapes due more to 

 its own inherent tendencies and less to the effect upon it of the 

 pressure of the uterine walls. 



(v) The thicker and more lasting these coats are, the more 

 marked are the intrinsic characters of the blastodermic vesicle, 

 and the longer deferred is the impression of characters due to 

 the physical effects of the uterine walls. 



For instance, both the rabbit and the dog have investing 

 coats. In the rabbit it is far thicker, so in the rabbit it is 

 found that the vesicle assumes a shape which can be best 

 accounted for by intrinsic causes, while in the dog, where 

 it is much thinner, the uterus seems to be the more potent 



