REPRODUCTION 



207 



The beginning of the maturation is shown by prepara- 

 tion for the reduction division. The chromatic substance 

 gathers into the usual number of chromosomes, which 

 form gemmini and take their usual position in the nuclear 



Germinal cell oocyte (ovule) at rest. 



Oocyte in process of maturation, showing the four 

 chromosomes, two (P and p) of paternal, and two 

 (M and m) of maternal origin. 



Conjugation of the chromosomes, the appearance cor- 

 responding with what is seen in spermatogenesis. 



Separation of the conjoined chromosomes and reduc- 

 tion division heterotype mitosis through the 

 movement of entire chromosomes to each polar field. 



Result of the reduction division, one large cell and one 

 tiny cell (polar body), each with two chromosomes. 



Homotype mitosis. Divisions of each chromosome 

 into two, and passage of each into a new cell. 



End-result, four cells each with the reduced number of 

 chromosomes; one cell, the ovum or gamete, being 

 large and functional ; the other three polar bodies 

 being functionless and abortive. 



FIG. 82. Oogenesis. Diagram showing the changes attending the maturation of 

 the ovum and the reduction division incidental to the formation of the female gamete. 

 As in spermatogenesis, oogenesis is accompanied by two cell divisions, one occurring 

 immediately after the other, the first or reduction division being by heterotype, 

 the second, by homotype mitosis. In all higher animals and plants, the effect is to 

 diminish the number of chromosomes in the gamete to one-half of the somatic 

 number. Unlike spermatogenesis, the four cells resulting from the divisions are 

 not of equal valence. One, the egg, is functional and is of large size, three are minute 

 and of no known value. Again a mammalian cell with four chromosomes is used 

 for convenience in the diagram. 



spindle. The gemmini then separate, each gemminus 

 turning to the pole of the cell opposite to its fellow. The 

 division, therefore, ends in the appearance of two cells, 

 each with one-half the somatic number of chromosomes. 



