REPRODUCTION 



195 



type mitosis, so that all of the four resulting cells contain 

 the reduced number of chromosomes. Again the division 

 lacks uniformity, the large cell again dividing into a large 

 and a small cell, and the small cell into two small cells of 



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. 



[omotype 

 into two, 



mitosis. Divisions of each chromosome 

 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. 



uniform size. The final outcome is four cells, of which 

 one, the ovum, is large, and three, the so-called polar 

 bodies, very small in comparison. The ovum seems to be 

 the only functionally active cell (gamete), the others are 



