230 EIOHARD ASSHETON. 



the paper, Hubrecht considers the trophoblast to be of 

 epiblastic origin, and he compares it to the outer layer of 

 epiblast of certain amphibians. He derives the mammalian 

 egg from a small egg like that of amphibians, with compara- 

 tively little yolk. 



Apart from all question of homology and mammalian de- 

 scent the term trophoblast seems to me most useful in 

 describing the events of mammalian development, and as such 

 1 have adopted it in my descriptions of the sheep and pig ; but 

 I do not adopt his hypothesis, because — 



(1) It offers no explanation of the mode of origin of the 

 trophoblast. On Hubrecht's hypothesis we should have ex- 

 pected a process of delamination, whereas in the sheep and 

 bat, where the two groups of cells are the most clearly defined, 

 it is an essentially horizontal division,, as in the meroblastic 

 eggs of birds and reptiles. 



(2) It does not account for the throwing off of the tropho- 

 blast cells over the embryonic area, which is so evident in 

 certain forms; which is the more surprising since in the Anura 

 (to the epidermic layer of epiblast of which these cells are 

 supposed to owe their origin) there seems to be permanent 

 fusion between the epidermic and nervous layers, as described 

 by myself (2). 



(3) It is not applicable to my specimens of the sheep. 



In order to make my observations accord with Hubrecht's 

 hypothesis it is necessary to suppose that the inner layer of 

 dark cells in my fig. 11 has been derived from the light- 

 coloured cells. The difference is so great that I think it is 

 almost impossible to ascribe such an origin to them. Or else 

 we must suppose that these cells subsequently pass into the 

 outer layer again after the stages of ligs. 12 and 15. On the 

 other hand, it seems easier to make the description and figures 

 of Tupaia agree with ray description of the sheep. 



Hubrecht's figures in his pi. 1 show only a small differ- 

 ence in colour between the inner cell and the outer wall. For 

 instance, in the figs. 19—25 there is very little evidence to 

 prove that none of the inner cells were derived from the outer 



