252 RICHARD ASSHETON. 



the closest connection between the walls of the blastocyst and 

 the mother before the formation of the placenta. 



In large animals, such as the sheep and pig, it is doubtful 

 whether there is any connection quite comparable to the 

 ectoplacenta or trager eating its way into the maternal 

 tissue. The energy is present, but shows itself in an enormous 

 growth of the vesicle, which no doubt answers the purpose of 

 acquiring nutriment, but in a rather different way. 



In many of the smallest mammals we find also great 

 diflFerences in the mode of formation of the amnion. For 

 instance, the amnion of Erinaceus, Mus, Cavia, Vespertilio, 

 differ much from each other and from the Sauropsidan type. 

 In the larger animals, Sus, Ovis, and in those smaller ones 

 which retain the zona radiata for some considerable time, 

 Lepus, Sorex, Talpa, the amnion formation is apparently more 

 like that of the bird. 



Do the smaller mammals really show us the more primitive 

 type of amnion development for the mammalia ? 



One is tempted to ask, did a continuance of the retardation 

 of the epiblast and acceleration of the hypoblastic mass with 

 its tendency to surround the former, lead to a curving upwards 

 of the edges of the epiblast, which, by means of their subsequent 

 expansion, gave rise to an amnion which is essentially different 

 from the amnion of the Sauropsida, — an amnion dependent 

 in no way on the force of gravity, and less on the action 

 of the internal fluid than in the Sauropsida, and therefore less 

 likely to be so much affected by the frequent alteration of 

 position which must occur in the mammalian uterus ? Such 

 an amnion formation is actually found in the case of Vesper- 

 tilio (Duval). 



On this view the free edges of the advancing amnion fold, 

 as described by Duval, would be both morphologically and 

 physiologically equivalent to the advancing free edges of the 

 epiblast of the blastoderm of the Sauropsidan egg. 



A hastening of this process might lead to the condition^in 

 Pteropus (52) or Cavia in which the amnion is from the first 

 appearance perfect. 



