THE GERM-CELLS. 



ii 



with the normal nutritive supply of the cell may bring about amitotic division. 

 While short life and early disappearance may often be regarded as 

 characteristics of the products of direct multiplication, it has been shown that 

 the normal mitotic cycle may be interrupted for a time, with restoration and 

 continuance of the usual process. Thus, when the dividing cells of a fresh- 

 water alga were placed in water, to which a small amount of ether had been 

 added, the cells ceased dividing mitotically and underwent division by the 

 direct method; on being transferred to pure water, the cells resumed their 

 mitotic division. In addition to the blastodermic cells of the embryo, 

 amitosis has been observed in the epithelium of the skin and bladder. 



ORIGIN AND DIFFERENTIATION OF THE CELLS. 



The body, with all its complex details, is the product of the differen- 

 tiation and specialization of cells which are the descendants of the fertilized 

 ovum. The latter represents the two parents, since the chromatin of the 



Corona radiata 



Zona pellucida 



Germinal vesicle (nu- 

 cleus) containing ter- 

 minal spot (nucleolus) 



Zone rich in 

 deutoplasm 



Zone poor in 

 deutoplasm 



FIG. 8. Human ovum from ripe Graafian follicle. X 160. (Nagel.) 



segmentation nucleus is contributed equally by the germ-cells, the sperma- 

 tozoon and the ovum. 



The ovum is formed within the female sexual gland, the ovary, where 

 it passes through all stages of development, from immaturity to maturation, 

 until finally liberated by rupture of the ovarian tissue. As a cell, the ovum 

 is interesting, since it possesses all parts of the typical cell, including a cell- 

 wall. These parts have long been designated by special names; thus, in the 

 nomenclature of the egg, the cytoplasm is called the vitellus or yolk, the 

 nucleus the germinal vesicle, the nucleolus the germinal spot, and the cell- 

 wall the oolemma or vitelline membrane. While the ova of birds and rep- 

 tiles are often of huge size, the yolk of the hen's egg corresponding to a 

 single cell, the. true ovum, the mammalian ova are much smaller and barely 

 visible with the unaided eye. The human ovum, when discharged from the 

 ovary, is about . 2 millimeter in diameter, spherical in form and composed 

 of cytoplasm containing innumerable yolk-granules. The latter, the repre- 

 sentatives of the abundant masses of nutritive material or deutoplasm stored 

 as the food-yolk in the bird's egg, are especially numerous in the vicinity of 



