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1606 A MANUAL OF ANATOMY 



disappearing. In spermatogenesis, on the other hand, though four 

 spermatids are formed at the same stage of cell-division as in 

 oogenesis, they are all equal in size. (2) The mature ovum under- 

 goes no further change, unless it becomes fertilized. Each sper- 

 matid, on the other hand, becomes transformed into an active 

 spermatozoon, capable of fertilizing a mature ovum. Spermato- 

 genesis may therefore be said to comprise one stage more than 

 oogenesis, but this additional stage is not one of cell-division, but 

 is simply the transformation of a spermatid into a spermatozoon. 



Reduction-Division. — This process consists in the reduction, at 

 each cell-division, of the nimiber of chromosomes or segments of the 

 chromatic spireme within the nucleus. In the male and female 

 germ-cells the chromosomes are arranged in groups of four, each 

 quadruple group being called a tetrad. When a tetrad is split into 

 two equal parts, two groups are formed, each containing two 

 chromosomes, and each of these pairs is called a dyad. The number 



' Division of ist Polar Body 



ist Polar ist Polar Spindle and 



Body Body -^ Elevation 



^ >\ \ 



' X' 



\ 



/ 



First Stage Second Stage Third Snge 



Fig. 662. — Extrusion of Polar Bodies (modified after Hertwig). 



of individual chromosomes, which is always an even number, is 

 constant and characteristic of each species of animal. 



Primary Oocyte (Ovum prior to Maturation). — ^The number of 

 single chromosomes in the germinal vesicle, for the purpose of illus- 

 trating reduction-division, may be assumed to be eight, these being 

 disposed as two tetrads. At the first unequal cell-division, or extru- 

 sion of the first polar body, each of these two tetrads is split into a 

 dyad, four dyads being thus formed. Two dyads go with the first 

 polar body, and two remain within the germinal vesicle.- Though, 

 therefore, the cell-division is unequal, the reduction and dis- 

 tribution'of the chromosomes is equal. When the first polar body 

 subdivides (during the extrusion of the second polar body) one 

 dyad goes with each subdivision. 



At the second unequal cell-division, or extrusion of the second 

 polar body, each of the two dyads within the germinal vesicle splits 

 into two chromosomes, and one chromosome of each dyad goes with 



