SEGMENTATION 



339 



large hollow sphere. The space thus left between the two 

 kinds of cells is called the segmentation cavity. Soon the sur- 

 rounding cells become attenuated (Rauber's 'cells) and dis- 



FIG. 96. Sections of the ovum of a rabbit, showing the formation ot 

 the blastodermic vesicle. (From Yeo after E. Van Beneden.) 



a, b, c, d,- are ova in successive stages of development; z.p., zona pellucida; 

 ect, ectomeres, or outer cells; ent, entomeres, or inner cells. 



appear. Their place, as a surrounding envelope, is taken 

 by some of the cells of the inner layer. This second sur- 

 rounding layer is the epiblast, or ectoderm; the surrounded 

 mass is the hypoblast, or entoderm. 



