130 THE DEVELOPMENT OF THE GAMETES OR SEX CELLS 



3. Nuclear Maturation of the Gametes 



Most of our information concerning the maturation of the nucleus pertains 

 to certain aspects of chromosome behavior involved in meiosis, particularly 

 the reduction of the chromosome number together with some activities of 

 "crossing over" of materials from one chromosome to another. But our infor- 

 mation is vague relative to other aspects of nuclear development. For example, 

 we know little about the meaning of growth and enlargement of the nucleus 

 as a whole during meiosis, an activity most pronounced in the oocyte. Nor 

 do we know the significance of nuclear contraction or condensation in the 

 male gamete after meiosis is completed. Therefore, when one considers the 

 nuclear maturation of the gametes, it is necessary at this stage of our knowledge 

 to be content mainly with observations of chromosomal behavior. 



a. General Description of the Chromatin Behavior During Somatic and 



Meiotic Mitosis 



As the maturation behavior of the chromatin components in the spermato- 

 cyte and oocyte are similar, a general description of these activities is given 

 in the following paragraphs. Before considering the general features and de- 

 tails of the actions of the chromosomes during meiosis, it is best to recall 

 some of the activities which these structures exhibit during ordinary somatic 

 and gonial mitoses. 



Cytological studies have shown that the chromosomes, in most instances, 

 are present in the nucleus in pairs, each member of a pair being the homologue 

 or mate of the other. Homologous chromosomes, therefore, are chromosomal 

 pairs or mates. During the prophase condition in ordinary somatic and gonial 

 mitoses, the various chromosomal mates do not show an attraction for each 

 other. A second feature of the prophase stage of ordinary cell division is that 

 each chromosome appears as two chromosomes. That is, each chromosome 

 is divided longitudinally and equationally into two chromosomes. At the time 

 when the metaphase condition is reached and the chromosomes become ar- 

 ranged upon the metaphase plate, the two halves or daughter chromosomes 

 of each original chromosome are still loosely attached to each other. However, 

 during anaphase, the two daughter chromosomes of each pair are separated 

 and each of the two daughter nuclei receives one of the daughter chromo- 

 somes. Reproduction of the chromatin material and equational distribution 

 of this material into the two daughter cells during anaphase is a fundamental 

 feature of the ordinary type of somatic and gonial mitoses. The two daughter 

 nuclei are thus equivalent to each other and to the parent nucleus. In this 

 way, chromosomal equivalence is passed on ad infinitum through successive 

 cell generations. 



On the other hand, a different kind of chromosomal behavior is found 

 during meiosis, which essentially is a specialized type of mitosis, known as 

 a meiotic mitosis. In one sense it is two mitoses or mitotic divisions with only 



