DEVELOPMENT OF THE SPERMATOZOA. 651 



organs, as it is modified for the development and growth of the 

 embryo : its function, however, can be performed by other organs, 

 as is seen in cases of extra-uterine foetation, when the ovum de- 

 velops in some unusual situation, such as in the Fallopian tube, 

 or in the abdominal cavity. 



The spermatozoa are formed by the cells lining the tubuli semi- 

 niferi of the testicle. These cells are cubical masses of proto- 

 plasm, which undergo rapid proliferation. The nuclei divide, 



FIG. 246. 



Section of the tubuli seminiferi of a rat. (Schafer.) a, Tubuli in which 

 the spermatozoa are not fully developed. 6, Spermatozoa more developed, 

 c, Spermatozoa fully developed. 



and from each part resulting from this division arises the head of 

 a spermatozoon, and the body is developed from the protoplasm 

 of the cell. The spermatic elements escape into the tubes, and 

 pass down the vasa deferentia into the vesiculse seminales, where 

 they either undergo a retrograde change or are cast out of the 

 body. 



The ovum arises from the differentiation of one of the cells of 

 the germ-epithelium covering the surface of the ovary. A group 

 of these cells, entering the periphery of the ovary, becomes there 



