EMBRY01 



tocytes by their subdivision give origin to four spermatozoa, and the secondary 

 oocyte and first polar body to four cells, the mature ovum and three polar bodies. 

 In the development of the spermatozoa, as in the maturation of the ovum, there 

 is a reduction of the nuclear chromosomes to one-half of those present in the 

 primary spermatocyte. But here the similarity ends, for it must be noted that 

 the four spermatozoa are of equal size, and each is capable of fertilizing a mature 

 ovum, whereas the three polar bodies are not only very much smaller than the 

 mature ovum but are incapable of further development, and may be regarded as 

 abortive ova. 





FERTILIZATION OF THE 'OVUM. 



Fertilization consists in the union of the spermatozoon with the mature ovum 

 (Fig. 8). Nothing is known regarding the fertilization of the human ovum, but 





1. Polar bodies 

 Female pronucleus 



Male pronucleus 



3. 



Female pronucleus 

 Male pronucleus 



5. 



Segmentation 

 nucleus 



2. 



Female pronucleus 

 Male pronucleus 



4. 



Fused pronuclei 



6. 



Segmentation 



nucleus 

 (commencing 



division) 



FIG. 8. The process of fertilization in the ovum of a mouse. (After Sobotta.) 



the various stages of the process have been studied in other mammals, and from 

 the knowledge so obtained it is believed that fertilization of the human ovum takes 

 place in the lateral or ampullary part of the uterine tube, and the ovum is then 

 conveyed along the tube to the cavity of the uterus a journey probably occupy- 

 ing seven or eight days and during which the ovum loses its corona radiata and zona 

 striata and undergoes segmentation. Sometimes the fertilized ovum is arrested 

 in the uterine tube, and there undergoes development, giving rise to a tubal preg- 

 nancy; or it may fall into the abdominal cavity and produce an abdominal preg- 

 nancy. Occasionally the ovum is not expelled from the follicle when the latter 

 ruptures, but is fertilized within the follicle and produces what is known as an 

 ovarian pregnancy. Under normal conditions only one spermatozoon enters the 

 yolk and takes part in the process of fertilization. At the point where the sperma 



