29^ THE HISTOPvY OF CEEATION. 



or yolk-membrane, which here bears the special name of 

 zona pellucida (d). The eggs of many lower animals 

 (for example of many Medusse) differ from this in being 

 naked cells, as the outer covering, or cell-membrane, is 

 wanting. 



As soon as the egg (ovulum) of the mammal has attained 

 its full maturity, it leaves the ovary of the female, in which 

 it originates, and passes into the oviduct, and through this 

 narrow passage into the wider pouch or womb (uterus). If, 

 meanwhile, the egg is fructified by the male seed (sperm), it 

 develops itself in this pouch into an embryo, and does not 

 leave it until perfectly developed and capable of coming 

 into the world at birth as a young mammaL 



The variations of form and transformations which the 

 fructified egg must go through within the uterus before it 

 assumes the form of the mammal are exceedingly remark- 

 able, and proceed from the beginning in man, in precisely 

 the same way as in the other mammals. At first the fructi- 

 fied egg of the mammal acts as a single-celled organism, 

 which is about to propagate independently and increase 

 itself; for example, an Amoeba (compare Fig. 2, p. 188). 

 In point of fact the simple egg-cell becomes two, by the 

 process of cell-division which I have previously described. 

 There arise from the single germinal spot (the small kernel- 

 speck of the original simple egg-cell) two new kernel-specks, 

 and then in like manner, out of the germinal vesicle (the 

 nucleus), two new cell-kernels. Then, and not until then, 

 does the globular protoplasma first separate itself by an 

 equatorial furrow into two halves, in such a manner that 

 each half encloses one of the two kernels, together with 

 its kernel-speck. Thus the simple egg-cell, within the 



