184 RIOHAED ASSHETON. 



The coat, however, is very much less firm, and offers 

 apparently less resistance than that of the rabbit, for, later, 

 after the vesicle has filled the cavity of the uterus the walls 

 very soon become moulded into the shape of the cavity of the 

 uterus (v. Heape, PI. 28, figs. 8 and 9). 



Of course it is necessary to take into consideration the fact 

 that the actual lumen of the uterus in the mole is very much 

 less than in the rabbit, and that therefore the developing 

 vesicle will be influenced by the resistance offered by the 

 muscular coat of the uterus at a time when the vesicle is much 

 smaller than in the case of the rabbit. 



Now to consider another case where we have pretty complete 

 records, i.e. the guinea-pig. 



During the early segmentation stage the ovum of the 

 guinea-pig is surrounded by a zona radiata. We find that in 

 the development of the early stages the embryo assumes just 

 the same form as in the mole or rabbit ; but after the embryo 

 has reached the uterus the zona is ruptured, and from this 

 moment the embryo is naked and goes through phases which 

 are certainly more like those of the rat than those of the 

 rabbit or mole. It can hardly be necessary to point out that 

 whereas at first the conditions were similar to those of the 

 rabbit or mole, from this moment they resemble more closely 

 those of the rat. 



In this question I have only gone into any details in the 

 case of the rabbit. But still, although I can only judge of the 

 other forms from either rather superficial personal examination 

 or from the writings of others in which this point has not been 

 given any special prominence, I think I may be allowed to 

 make a few general remarks upon such other forms in which 

 marked divergence of shape and structure occurs in the 

 method of formation of the blastodermic vesicle and germinal 

 layers. 



Amongst the Carnivora, from observations on the dog by 

 Bischoff and Coste, and on the cat by Fleischmann, it seems 

 clear that the condition most nearly resembles the rabbit. 



The ova are in all these cases invested with a firm coat, 



