THE EARLY DEVELOPMEXT OF THE MAKSUPIALTA. 99 



in eafly cleavage stages, owing to differences in the cliaracters 

 and staining reactions of tlieir cells, and in such cases there 

 is definite evidence of the occurrence of a process of overgrowth 

 or epiboly, whereby one group gradually grows round and 

 completely envelops the other, so that in the completed 

 morula a distinction may be drawn between a central cell- 

 mass and a peripheral or enveloping la^-er (rabbit, Van 

 Beneden ; sheep, Assheton). In other cases^ where it has 

 been impossible to recognise the existence of these two 

 distinct cell-groups in the cleavage stages, we nevertheless 

 find, either in the completed morula or in the blastocyst, that 

 a more or less sharp distinction may be drawn between an 

 enveloping layer of cells and an internally situated cell-mass 

 (inner cell-mass). 



E. van Beneden, in his classical paper on the development 

 of the rabbit, published in 1875, was the first to recognise 

 definitely the existence of two categories of cells in the 

 Segmenting egg of the Eutherian mammal. In this form he 

 showed how in the morula stage a cap of lighter blastomeres 

 gradually grows round and envelops a mass of more opaque 

 cells by a pi'ocess of overgrowth or epiboly. In his more 

 recent and extremely valuable paper on the development of 

 Vespertilio ('99), he again demonstrated the existence of two 

 groups of blastomeres as well in the segmenting egg as in the 

 completed morula, but failed to find evidence of epiboly in all 

 cases. Nevertheless he holds fast to the opinion which he 

 expressed in 1875 : " Que la segmentation s'accompagne, chez 

 les Mammifei-es placentaires, d'un enveloppement progressif 

 d'une partie des blastomeres par une couche cellulaire, qui 

 commence a se differencier des le debut du developpement," 

 and states that " dans tons les oeufs arrives a la fin de la 

 segmentation et dans ceux qui moutraient le debut de la 

 cavite Blastodermique i'ai constamment rencontre une couche 

 peripherique complete, entourant de toutes parts uu amas 

 cellulaire interne, bien sypare de la couche enveloppante." 

 The latter layer he regards as corresponding to the extra- 

 embryonal ectoderm of the Sauropsida, and points out that 



