124 



SIMPLE HISTOLOGICAL ELEMENTS. 



Fig. 62. 



Segmentation of the vitellus 

 (yolk) of the mammalian ovum. 

 A. Its first division into two 

 halves. B. Subdivision of each 

 half into two. c. Further sub- 

 division, producing numerous 

 segments. 



cleoli, and are developed either in a plasma 

 or within pre-existing cells. 



In the fecundated ovum the formation of 

 cells is preceded by a peculiar process the 

 cleavage or segmentation of the yolk which 

 is introductory to an 'endogenous cell-deve- 

 lopment. The nucleus of the parent cell (i. e. 

 the germinal vesicle), Fig. 55. disappears after 

 fecundation, and the granules become dispers- 

 ed and fill the whole cell. Another nucleus is 

 then developed around a nucleolus, and the 

 yolk forming around it, in a globular form, 

 constitutes the first cleavage-mass. From 

 this nucleus two others are formed, as before 

 described, and then two cleavage-masses re- 

 sult; and thus the multiplication proceeds 

 till the cavity of the yolk is filled with small 

 globules. Lastly, the latter, either together 

 or in successive layers, are surrounded with 

 investing membranes, and thus become per- 

 fect cells the parent cell still persisting, 

 and inclosing them. Figs. 62 and 63 illus- 

 trate this process, and also show spermato- 

 zoids on the outer surface of the vitellus or 

 yolk. 



Generally, the development of a large 

 number of cells within a single parent cell 

 occurs only in structures of a rapid growth, 



Fig. 63. 



Segmentation of the yolk latter stages. A. The ''mulberry-mass" from the minute subdivision 

 of the vitelline spheres. At B it has come into contact with the vitelline membrane, against which 

 the spherules are flattened. 



