CHAPTER III 

 THE MECHANISM OF SEGREaATION 



One of the most secure generalizations of modem work 

 on the cell is that every cell of the individual contains a 

 constant number of self-perpetuating bodies (called chro- 

 mosomes), half of which are traceable to the father and 

 half to the mother of the individual. No matter how 

 specialized cells may be, they contain the same number 

 of chromosomes. Equally important is the fact that after 

 the eggs of the female and the sperm-cells of the male 

 have passed through the ripening or maturation divisions 

 the number of chromosomes is reduced to half.^ Lastly, 

 there is convincing evidence that the reduced number of 

 chromosomes is brought about as the result of a separa- 

 tion of such a kind that each mature germ-cell gets only a 

 paternal or a maternal member of each chromosome pair. 



The reduction takes place in the female at the time 

 when the polar bodies are given off from the egg ; and in 

 the male just prior to the formation of the spermatozoa. 

 A characteristic process is seen in the oogenesis and sper- 

 matogenesis of the nematode worm Ancyracanthus cysti- 

 dicola (a parasite in the swim-bladder of fresh- water 

 fishes) described by Mulsow. The young eggs contain 

 twelve chromosomes (Fig. 14, a). As the result of the 

 later union of these twelve in pairs, six short threads 

 appear in the nucleus of the egg just before it extrudes its 

 polar bodies. The threads contract to six short rods 

 (split in two planes at right angles to each other), the 

 tetrads (Fig. 14, c). With the dissolution of the nuclear 

 wall these tetrads are set free in the protoplasm, and a 

 spindle develops about them (Fig. 15, a). They pass to 

 the equator of the spindle, and there dividing lengthwise, 



' Exceptions occur in certain cases of parthenogenesis. 



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