246 ANIMAL BIOLOGY 



regard tetrad formation since it in no wise alters the basic results 

 attained by spermatogenesis or oogenesis. 



4. Oogenesis 



The maturation of the egg, as already intimated, follows the 

 same plan as that of the sperm, and the reduction of the chromo- 

 somes is the same. Such modifications as occur are related to the 

 fact that the egg is usually a relatively large, passive cell stored 

 with nutritive materials for use during the developmental process, 

 while the sperm is among the smallest of cells — essentially a nu- 

 cleus surrounded with a delicate envelope of cytoplasm. Accord- 

 ingly it is only necessary to emphasize that the growth period of 

 egg formation, in which the oogonium becomes transformed 

 into the primary oocyte, is characterized by a much greater 

 increase in size than is the case in the corresponding period in 

 spermatogenesis; and that the following two cell divisions (mat- 

 uration divisions) involving chromosome reduction result in very 

 unequal division of the cytoplasm. Thus one secondary oocyte 

 is very large, while the other is a tiny cell termed the first polo- 

 cyte. 



Both the large secondary oocyte and first polocyte now divide 

 again; the former giving rise to a large cell, the mature egg, and 

 a tiny second polocyte; while the first polocyte divides equally 

 to form two polocytes. In this way arise the four cells, compa- 

 rable to the four sperm in spermatogenesis, each with half the 

 somatic number of chromosomes. But only one of these, the egg, 

 functions as a gamete. The three polocytes, although possess- 

 ing a similar chromosome complex, are sacrificed in providing one 

 cell, the egg, with its special cytoplasmic equipment. The polo- 

 cytes get just enough cytoplasm to be regarded as cells, and soon 

 degenerate and disappear. (Figs. 165, 166.) 



Such is the outline of the essentials of spermatogenesis and 

 oogenesis in animals ; processes which involve at one stage a modi- 

 fication of ordinary mitosis to give each gamete half the somatic 

 number of chromosomes characteristic of the species. It is clear 

 that this is not merely a mass reduction of chromatin material, 

 but is a separation and segregation after synapsis of definite chro- 

 matin entities, the chromosomes, so that the gametes receive the 

 reduced number. 



