OUR EMBRYONIC DEVELOPMENT. 63 



The human ovum, like that of all other animals, is 

 a single cell, and this tiny globular egg-cell (about the 

 120th of an inch in diameter) has just the same charac- 

 teristic appearance as that of all other viviparous 

 organisms. The little ball of protoplasm is sur- 

 rounded by a thick, transparent, finely reticulated 

 membrane, called the zona pellucida ; even the little, 

 globular, germinal vesicle (the cell-nucleus), which is 

 enclosed in the protoplasm (the cell-body), is of the 

 same size and the same qualities as in the rest of the 

 mammals. The same applies to the active sperma- 

 tozoa of the male, the minute, thread-like, ciliated 

 cells of which millions are found in every drop of the 

 seminal fluid ; on account of their life-like movements 

 they were previously taken to be forms of life, as the 

 name indicates (spermatozoa = sperm-animals) . More- 

 over, the origin of both these important sexual cells 

 in their respective organs is the same in man as in 

 the other mammals ; both the ova in the ovary of the 

 female and the spermatozoa in the spermarium of the 

 male arise in the same fashion — they always come 

 from cells, which are originally derived from the 

 coelous epithelium, the layer of cells which clothes the 

 cavity of the body. 



The most important moment in the life of every 

 man, as in that of all other complex animals, is the 

 moment in which he begins his individual existence ; 

 it is the moment when the sexual cells of both parents 

 meet and coalesce for the formation of a single simple 

 cell. This new cell, the impregnated egg-cell, is the 

 individual stem-cell (the cytula), the continued segmen- 

 tation of which produces the cells of the germinal 

 layers and the gastrula. With the formation of this 

 cytula, hence in the process of conception itself, the 



