228 EIOHARD ASSHETON. 



suggested that the phenomenon is capable of interpretation in 

 some such way as that indicated by my Plate 18, 



The obvious difficulties which arise from this last hypothesis 

 may be stated thus : 



(i) The alternative explanations given of individual de- 

 developmental histories by other authors. 



(ii) The double-layered condition of the hypoblast. 



(iii) (a) The inference that the chorion of placental mammals 

 is formed by the fusion of allantois and extra-embryonic hypo- 

 blast (trophoblast), whereas in birds and reptiles the corre- 

 sponding membrane is formed by the fusion of the allantois 

 and extra-embryonic epiblast, with^ of course, in each case the 

 intervening raesoblast. (b) Although the true amnion may in 

 every case be either wholly or partly formed of true epiblast, 

 in all cases the false amnion would appear to be formed of 

 hypoblast. 



These difficulties are discussed below. The chief features of 

 my hypothesis which seem to me to be most likely to commend 

 it to the attention of embryologists may be stated thus : 



1. It is an explanation of the early differentiation of the 

 segments into two groups of cells, which avoids the difficulty 

 which attends that of van Beneden, 



2. It is an explanation of the van Beneden blastopore. 



3. It suggests a reason for the rejection of the Bauber layer 

 cells by the embryonic epiblast, which is so evident a pheno- 

 menon in some cases. 



4. It demands no change in function of the two primary 

 layers during the substitution of the yolk granules by maternal 

 fluids. The hypoblast cells maintain their function as a nutri- 

 tive organ, entering into direct connection with the maternal 

 tissues, in nearly all animals, in the various forms of trager, 

 ectoplaceuta, and villi, and thereby supporting the embryo 

 until the placenta is established. 



5. The morphological relation of tlie epiblast and hypoblast 

 and the cavity from which the future gut is formed agree on 

 this explanation absolutely with the conditions of the Saurop- 

 sidan and Monotreme eg^. 



