MATURATION OF GAMETES 133 



(fig. 67, second meiotic division; fig. 69). As a result of these two divisions, 

 each daughter cell thus contains the haploid or reduced number of chromo- 

 somes in the monad (monoploid) condition (fig. 67, spermatid or egg). Meiosis 

 or chromatin diminution is now an accomplished fact. 



It is to be observed, therefore, that the meiotic phenomena differ from 

 those of ordinary mitosis by two fundamental features: 



(1) In meiosis there is a conjugation (synapsis) of homologous chromo- 

 somes during the prophase stage, and while synapsed together each 

 of the homologues divides equationally; and 



(2) following this single prophase of peculiar character, two divisions 

 follow each other, separating the associated chromatin threads. 



While the meiotic prophase is described above as a single prophase pre- 

 ceding two metaphase-anaphase chromosome separations, it is essentially a 

 double prophase in which the process of synapsis acts to suppress one of 

 the equational divisions normally present in a mitotic division; a synapsed 

 or double chromosome, therefore, is substituted for one of the longitudinal, 

 equational divisions which normally appears during a somatic prophase. It is 

 this substitution which forms the basis for the reduction process, for two 

 mitotic divisions follow one after the other, preceded by but one equational 

 splitting, whereas in ordinary mitosis, one equational splitting of the chromo- 

 somes always precedes each mitotic division. 



b. Reductional and Equational Meiotic Divisions and the Phenomenon of 



Crossing Over 



In the first meiotic division (i.e., the first maturation division), if the two 

 chromatids which are derived from one homologous mate of the tetrad are 

 separated from the two chromatids derived from the other homologous mate 

 the division is spoken of as reductional or disjunctional. In this case the two 

 associated chromatids of each dyad represent the original chromosome which 

 synapsed at the beginning of meiotic prophase (fig. 67, tetrads B and C, 

 first meiotic division). If, however, the separation occurs not in the synaptic 

 plane but in the equational plane, then the two associated chromatids of each 

 dyad come, one from one synaptic mate and one from the other; such a 

 division is spoken of as an equational division (fig. 67, tetrad A, first meiotic 

 division). There appears to be no fixity of procedure relative to the separation 

 of the tetrads, and great variability occurs. However this may be, one of the 

 two meiotic divisions as far as any particular tetrad is concerned is disjunc- 

 tional (reductional) and the other is equational, at least in the region of the 

 kinetochore (see p. 135 and fig. 70). If the first division is reductional, the 

 second is equational and vice versa. Disjunction in the first maturation division 

 is often referred to as pre-reduction, while that in the second maturation di- 

 vision is called post-reduction. 



