. PROCESSES RELATED TO SEXUAL REPRODUCTION 229 



such cases is the same, since the second maturation is then Hke 

 any other mitotic division, with the chromosomes dividing length- 

 wise and producing the same number as in the parent cell. After 

 this second maturation or " reducing " division, a spermatozoon, 

 with this reduced or haploid number of chromosomes, is pro- 

 duced from each of the four cells by changes in the cytosome and 

 condensation of the nucleus (Fig. 115). The essential significance 

 of spermatogenesis is, therefore, the reduction in the number of 

 chromosomes of the mature spermatozoon to one-half the number 

 characteristic of the spermatogonia and all other cells of the 

 animal. 



Maturation of the Female Germ Cells: Oogenesis. — The matura- 

 tion of the female germ cells, or oogenesis (Fig. 114), is com- 

 parable with that of the male cells so far as the nuclear changes 

 are concerned, although only one of the four cells that are formed 

 by the two maturation divisions becomes a functional ovum. 

 The other three are small cells with the same number of chromo- 

 somes as the ovum, but with a minimum amount of cytoplasm. 

 As these are formed at the so-called animal pole of the ovum 

 and can be recognized as minute globules (c/. Fig. 148, p. c07), 

 they were called the polar bodies by the earlier embryologists 

 before their true significance was ascertained. This difference in 

 the maturation of the male and female germ cells may be regarded 

 as a device v.hereby the cytoplasmic materials that might have been 

 distributed to four equivalent cells are given instead to one of the 

 four, which thus obtains a proportionately greater amount of the 

 nutrient material that is utilized in the early stages of develop- 

 ment. The polar bodies might, therefore, be described as abortive 

 ova that come to naught, because one of their fellows receives the 

 cytoplasm that might have been theirs. Another difference in the 

 development of the ovum and spermatozoon is the amount of 



oogonia or early germ cells of the female. In synapsis the autosomes unite in pairs and 

 likewise the " X " chromosomes. The two maturation divisions produce from each oogonium 

 a single ovum and three abortive cells, the polar bodies (c/. Fig. 148, p. 307 and Fig. 211. 

 p. 4012). In the first maturation division there is no change in the number and relation- 

 ships of the chromosomes, because each one splits lengthwise as in typical cases of mitosis. 

 In the second maturation division the pairs of autosomes, which united in synapsis, separate 

 and pass without further division to the resulting cells. In this manner the ovum and 

 each polar body receives one member of each pair of autosomes and one "X" chromosome. 

 The fertilization of an ovum by a spermatozoon that possesses an "X" chromosome 

 gives an oosperm or zygote with two "X" chromosomes and hence a female. Fertilization 

 with a spermatozoon that does not possess an "X" chromosome gives a njale (<■/. p. 448). 



