THE CELL 59 



fact that in the higher plants no ccntrosomc can be demonstrated during any 

 stage of mitosis, and also that in some cases the chromosomes divide without 

 previous division of the centrosome. Between mitotic periods the centrosome 

 with or without its aster may remain as an integral part of the resting cell. It 

 may, on the other hand, entirely disappear during the resting stage. 



Branca calls attention to the fact that the centrosome is an organ of no "con- 

 stancy, permanence, or specificity," (i) that in certain cells it is impossible to 

 demonstrate a centrosome at any time; (2) that when a centrosome has been 

 demonstrated for a certain type of cell, it cannot always be found even in that 

 type of cell; (3) that certain parts of the centrosome, the rays, are cytoplasmic, 

 while another part, the centriole, reacts like nuclear material (chromatin). 

 He concludes that the centrosome is a portion of the protoplasm temporarily 

 differentiated for a specific function; not unlike the basal filaments which appear 

 temporarily in gland cells when they l^ccome active, both being functional forms 

 of protoplasm which can succeed each other as the cell changes the character 

 of its function." 



It is through the above-described process of cell-division that new 

 cells are produced to replace those worn out as a result of their labors 

 or destroyed by injury. It is through the same process that the vast 

 number of cells which make up the adult body are developed from 

 one original cell — the ovum. Such powers of evolution are not, 

 however, inherent in the ovum itself, but, in sexual reproduction, 

 are acquired only after its union with germinal elements from the 

 male. This union of male and female germinal elements is known as 

 fertilization of the ovum. 



Fertilization of the Ovum 



Prior to and in preparation for fertilization, both male and female 

 cells must pass through certain changes. These are known as matu- 

 ration of the spermatozoon on the male side (p. 350) and of the ovum on 

 the female (p. 363). 



The spermatozobn (Fig. 11) is developed from a cell of the sem- 

 iniferous tubule of the testis. The nucleus of this cell so divides its 

 chromosomes that each spermatozobn contains just one-half the num- 

 ber of chromosomes characteristic of cells of the species. These are 

 contained in the head of the spermatozoon, which thus represents the 

 nucleus of the male sexual cell, the middle piece probably containing 

 the centrosome, the tail piece the remains of the protoplasm. 



The nucleus of the ovum or germinal vesicle also passes through a 

 series of changes by which it loses one-half its chromosomes. The 

 germinal vesicle or nucleus of the ovum first undergoes mitotic 



