THE EAKLY DEVELOPMENT OF THE MAKSUPIALIA. 31 



the poles, in optical section (figs. 20, 21), they appear tri- 

 angular with rounded corners and centrally directed auices. 

 The space occupying the polar diameter, which they enclose 

 is the cleavage cavity. The blastomeres are now somewhat 

 less opaque than those of the 2-celled stage, so that their 

 nuclei, excentrically situated nearer their upper ends and 

 enclosed in the central granular zone of the cytoplasm, can 

 now be faii'ly distinctly made out in the fresh egg. 



The arrangement of the blastomeres at this stage is 

 exceedingly characteristic, and is identical with that of the 

 blastomeres in the corresponding stage of Amphioxus or the 

 frog, but is quite different from that normal for the 4-celled 

 stage of the Eutheria. They lie disposed radially or meri- 

 dionaily around the polar diameter, occupied by the cleavage 

 cavity, their thicker upper ends partially surrounding the 

 yolk-body. Selenka figures a precisely similar arrangement 

 in his 4-celled stage of Didelphys, so that we may conclude 

 it holds good for the Marsupials in general. 



AVhilst, then, in Marsupials the first two cleavage planes are 

 vertical or meridional, and at right angles to each other, and 

 the first four blastomeres are arranged radially around the 

 polar diameter (radial type of cleavage), in the Eutheria 

 such is never the case, at all events normally, so far as is 

 known. In the Eutheria the first four blastomeres form, or 

 tend to form, a definite cross-shaped group, as the result 

 apparently of the independent division of the first two blasto- 

 meres in two different planes at right angles to each other, 

 the division planes being meridional in the one, equatorial 

 in the other. ^ This pronounced difference in the spatial 

 relations of the first four blastomeres in the Metatheria and 

 Eutheria is a feature of the very g-reatest interest and im- 

 portance, since it is correlated with and in part conditions 

 the marked dissimilarity which we meet with in the later 

 developmental occurrences in the two groups, in particular 

 in the mode of formation of the blastocyst in the two. 



' Compare in this connection Assheton's remarks ('09, pp. 232-233), 

 which have appeared since this chapter was written. 



