44 



THE EARLY DEVELOPMENT OF MAMMALS. 



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. n, 5.2).* The inner accumulation of 

 cells is designated as the inner mass, i.m. Figure 10 represents a rabbit ovum of 

 about seventy hours, according to the observations of van Beneden. 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 continuous. In the 



FIG. 8. OVUM OF A BAT (VESPERTILIO MURINA) 

 WITH FOUR SEGMENTATION SPHERES. (After 

 van Beneden and Julin.) 



X300 



FIG. 9. OVUM OF A VIRGINIAN OPOSSUM, WITH 



FOUR SEGMENTS. 



.p.g } Polar globules, x, Coagulated material, z, Zona 

 pellucida. (After Emil Selenka.) 



next stage (Fig. n) we find that the ovum has become larger by the appearance 

 of a cavity in its interior. This cavity appears between the inner mass, i.m, and 

 the subzonal layer, but at one side the inner mass remains adherent to, and closely 

 connected with, the subzonal layer. We now have reached the stage in which the 

 developing ovum may be designated as the blastodermic vesicle. 



As to the interpretation of the parts, it is probable that the subzonal layer 

 is ectoderm, and that the central cells of the inner mass are also ectodermal and 

 share in forming the embryonic shield, and finally that the superficial cells of the 

 inner mass (i.e., those next the cavity of the vesicle) are entodermal. At the stage 

 we have now reached the blastodermic vesicle has a large part of its walls formed 

 by the subzonal layer only, so that we call this the stage of the one-layered blasto- 

 dermic vesicle. 



* The subzonal layer is termed trophoblast by A. A. W. Hubrecht, and is held by him to be a special embryonic 

 structure, developed in order fo establish special relations between the developing ovum and the walls of the uterus 

 to secure the nutrition of the former. It has seemed best to present a purely objective account of the facts without 

 entering into a discussion of the very interesting interpretations proposed by Hubrecht. 



