PROCESS OF FERTILIZATION. 



73 



Centrosome. 



Female pro- 

 nucleus. 



Chromo- 

 somes of 

 egg-nu- 

 cleus. 



Chromo- 

 somes of 

 male pro- 

 nucleus. 



Centrosome. 



Fig. 34- 



Fig. 35- 



Chromosomes 

 from egg-nu- 

 cleus. 



'."}-- Chromosomes 

 from sperm- 

 nucleus (male 

 pronucleus). 



-- Centrosome. 



Fig. 36. 



Figs. 34-36. Diagrams of the process of fertilization, after Boveri. 

 Figure 34, from the spirems in the pronuclei, chromosomes have been formed. The 

 centrosphere has divided. Figure 35, the double chromosomes of the two pronuclei lie in 

 the equatorial plane of the ovum. Figure 36, the ovum has divided. Chromosomes 

 from the male and female elements are seen in equal numbers in both daughter nuclei. 



(spermatozoa). In short, there is here an absence of structures 

 analogous to the polar bodies, which degenerate after maturation 

 of the ovum. 



The spermatozoa are flagellate cells. The head consists prin- 

 cipally of nuclear substance, to which is added a smaller middle- 

 piece containing, according to the investigations of Pick, the centro- 

 some. These two portions of the male sexual cell, the head- and 

 middle-piece, are the most important, and are exclusively con- 

 cerned in fertilization, the flagellum or tail playing no part in this 

 process. 



The spermatozoon usually penetrates the ovum after the first 

 polar body has been extruded. The tail disappears during this 

 process, being either left at the periphery of the egg or dissolved in 

 the protoplasm. From this time the head represents the so-called 

 male pronucleus t and the middle-piece the centrosome. From this 

 stage the male pronucleus undergoes changes, the first of which 

 consists of a loosening of the chromatin. Chromatin granules are 



