OEIGIN OF CELLS. 



25 



turning to surround the central hypoblasts spheres, both sets continuing 

 to subdivide, until Anally the ovum consists of an almost solid mass of 

 hypoblasts spheres surrounded by a layer of epiblastic cells. 



When the process of segmentation is complete the epiblastic cells 

 are clear and have an irregularly cubical form, while the hypoblasts 

 cells are polygonal and granular and somewhat larger than the epi- 

 blastic cells. 



The blastodermic vesicle next forms. This results from the forma- 

 tion of a narrow cavity between the epiblast and hypoblast, which 

 increases in size until it entirely separates these two layers, except at the 

 point where the blastoderm was last in forming (the blastopore). As 

 the cavity increases in size the ovum also enlarges, so that soon it exists 

 in the form of a large vesicle, formed of a thin wall of a single layer of 



Fig. 15.— Optical Sections of a Rabbit's Ovum at Two Stages Closely 

 Following Upon the Segmentation, after E. Van Beneden. 

 {Balfour.) 



EP, epiblast ; HY, primary hypoblast ; BP, Van Beneden's blastopore. The shading of the epiblast 

 and hypoblast is diagrammatic. 



cells, — the epiblastic cells, — with a large cavity, the hypoblasts cells 

 forming a small, ventricular mass attached to the inner side of the epi- 

 blastic cells (Fig. 16). The ovum of the rabbit has now increased in 

 size from 0.09 mm., its size at the close of segmentation, to about 0.28 

 mm. It is inclosed by the vitelline membrane and a mucons layer 

 deposited by the walls of the Fallopian tube. 



As the vesicle continues to enlarge, the hypoblasts cells spread out 

 beneath the epiblast, though remaining thicker in the centre than at the 

 edges, where the cells still possess the power of amoeboid movement. 

 The central, thicker portion, which is the commencement of the embryonic 

 area, forms an opaque, circular spot on the blastoderm. 



The primitive hypoblast now becomes divided into two layers, the 

 lower continuous with the peripheral hypoblast and formed of flattened 

 cells, while the upper is formed of small, rounded elements,— the meso- 



