68 



THE EARLY DEVELOPMENT -OF MAMMALS. 



duced next, but it may very well be that there is really a three-cell stage preced- 

 ing the four-cell stage of which two figures are presented. The first of these 

 (Fig. 13) represents the four-cell stage of the ovum of a bat, and the second (Fig. 

 14) represents the four-cell stage of the ovum of the Virginian opossum. That 

 of the bat resembles the picture which we obtained from a number of animals, 

 such as the rabbit, the guinea-pig, the dog, and others. That of the opossum 

 differs so much from anything known in other mammals that it may be ques- 

 tioned whether it is entirely normal. In the mouse the zona is much thinner 

 and assumes an irregular form, adapting itself to the pressure of the single 

 spheres. 



After the four-cell stage, the segmentation proceeds apparently with con- 



'■■^.. 



300 



Fig. 13. — Ovum of a Bat (Vespertilio Murina) 

 WITH Four Segmentation Spheres. — [After 

 van Beneden and Julin.) 



Fig. 14. — Ovum of a Virginian Opossum, with 

 Four Segments. — {After Emit Selenka. ) 



siderable irregularity, but we are soon able to see that the cells are grouping 

 themselves into an uninterrupted external layer and an internal accumulation of 

 cells. The outer layer is in contact, or nearly in contact, with the zona radiata, 

 and may, therefore, be termed the subzonal layer (Fig. 16, s.z). The inner 

 accumulation of cells is designated as the inner mass, i.w. Fig. 15 represents a 

 rabbit ovum of about seventy hours, according to the observations of van Bene- 

 den. He represents the subzonal layer, Ec, as interrupted at one point, where one 

 of the cells of the inner mass, i.m, is exposed. It is probable, however, that van 

 Beneden is in error in regard to this, and that the subzonal layer is really con- 

 tinuous. In the next stage (Fig. 16) we find that the ovum has become larger 

 by the appearance of a cavity in its interior. This cavity appears between the 



