THE GERM-CELLS 43 



ovum. It was only about 1875, with the recognition of 

 the cell-structure of the sex-elements, that the correct 

 interpretation of fertilization was found in the union of the 

 nuclear substance of the male and female sex-cell. 



The spermatozoon (generally only one) as the active 

 element enters the ovum, which has already gone through 

 its ripening process, and possesses, to take the same ex- 

 ample of Ascaris megalocepkala, as instanced before, two 

 chromosomes. The nuclear substance of the sperm-cell 

 then changes its appearance. It becomes pale, grows in 

 size, and its network of chromatin transforms itself into 

 two chromosome loops, the same also taking place with the 

 chromatin of the ovum. Simultaneously the centrosome 

 introduced with the spermatozoon has doubled (the 

 centrosome of the ovum generally plays no role, and dis- 

 appears), and forms a double aster ; while the two " pro- 

 nuclei," as the male and female nuclei are now called, 

 approach each other in order to coalesce and form a single 

 " segmentation nucleus." This segmentation nucleus, being 

 formed by the union of male and female nucleus, therefore 

 now contains the hereditary substance of both germ-cells, 

 maternal as well as paternal, and once more possesses four 

 chromosomes — two from the father and two from the 

 mother. What has been formed is, indeed, the mother- 

 cell, from which the new individual arises in the ordinary 

 manner of cell-division — that is, the chromosomes split 

 lengthwise, wander to the opposite poles, and surrounding, 

 themselves with the separated halves of the cell-body, form 

 two new daughter-cells, each possessing, again, four chromo- 

 somes — namely, two from the father and two from the 

 mother. The daughter-cells repeat this process of division, 

 and by continued subdivision of the same kind the organism 

 is finally built up. Thus is brought about the mingling of 

 the parental qualities in the mother-cell and their equal 

 distribution throughout the line of the descendant body- 

 cells. 



It may seem from the description of these facts as if 



