254 RICHARD ASSHETON. 



plete, and in the absence of a more extended research into 

 the process of segmentation of the ovum of the Marsupialia 

 and Monotremata it is useless to discuss at any length the 

 present hypothesis with regard to those groups of mammals. 

 As far as we can gather from the little which Caldwell 

 and Semon have published concerning the segmentation of 

 the Monotreme egg, we must expect that the eggs of these 

 mammals resemble those of the Sauropsida, and that in them 

 the epiblast does grow round the yolk, producing a double- 

 walled sac consisting of an outer layer of epiblast, and an 

 inner layer of large yolk-laden hypoblast, as described by 

 Hill and Martin (31) in a specimen obtained from the uterus 

 of an ornithorhynchus, in which seventeen mesoblastic somites 

 were present, How this sac is formed we do not know. 



Whether the Marsupial development resembles that of the 

 Monotreme or that of the Placentalia more closely cannot at 

 present be decided. 



Whichever view is taken of Selenka's description of the 

 opossum, many obvious difficulties remain, for the solution of 

 which no satisfactory suggestion can as yet be offered. If, as 

 I believe, the development of the placental mammals shows us, 

 the protomammalian had a large-yolked egg, the Monotreme's 

 development would be of the greatest possible interest. 

 Hubrecht has, in the very interesting paper to which I have 

 frequently referred, most ably advanced the opposite view 

 that the protomammalian egg was a small egg of the amphi- 

 bian type, and that the Monotreme is an aberrant offshoot. If 

 I prefer to support the other view I am only adhering 

 to the opinion expressed by Balfour in his great work on 

 ' Comparative Embryology,' 2nd ed., p. 189 : — " The features 

 of the development of the placental Mammalia receive their 

 most satisfactory explanation on the hypothesis that their 

 ancestors were provided with a large-yolked ovum like that of 

 the Sauropsida. . . . The embryonic evidence of the common 

 origin of Mammalia and Sauropsida, both as concerns the 

 formation of the layers and of the embryonic membranes, is as 

 clear as it can be. The only difficulty about the early 



