586 A. A. W. HUBKECHT. 



restrict myself to a point of very fundamental importance on 

 which Assheton's and my own views are diametrically 

 opposed to each other, ever since 1898. If the new facts 

 which I bring forward in this paper should be convincing 

 enough to change the minds of those who feel inclined — 

 following Assheton's example — to look upon the ti-ophoblast 

 as hypoblastic, I have no doubt that my proposal to exclude 

 from the phylogeny of Eutherian mammals any ancestor who 

 deposited megalecithal eggs, like the Sauropsids and the 

 Ornithodelphia, will find a more easy acceptance on their 

 part. 



Assheton's reasons for considering the ti-ophoblast as an 

 essentially entodermal foetal envelope were first developed in 

 1898, in his article on " The Segmentation of the Ovum of 

 the Sheep" (' Quart. Journ, Micr. Sci.,' vol.41). Plate 18 

 of that article presents us with a series of diagrams most 

 delicately shaded in red and blue, which were meant to 

 explain the mutual relations of trophoblast, epiblast, and 

 hypoblast in ten different genera of mammals, and to compare 

 them with the Sauropsidan arrangement. 



These diagrams have not found favour with later authors 

 on this subject, and have been taken no notice of in Hertwig's 

 extensive ' Eutwickelungsgeschichte,' in three volumes. At 

 that time I refrained from entering into any polemical dis- 

 cussion, considering that later observations would show the 

 untenability of Assheton's ingenious but unsatisfactory 

 generalisation. In writing his latest article Assheton has, 

 however, allowed himself to come too strongly under the 

 influence of his own hypothesis of twelve years' standing. I 

 see no necessity for entering upon any detailed discussion 

 concerning the numerous and different arguments which have 

 led other embryologists as well as myself to reject that 

 hypothesis of Assheton's now that new facts have come to 

 light concerning the very earliest segmentation stages of 

 Galeopithecus. This very archaic genus may be looked upon 

 as a derelict representative of a group that in earlier 

 geological epochs gave rise to the modern bats. There are 



