MALE REPRODUCTIVE CELLS 42$ 



matocytes divide again, producing four spermatids which each possess 

 one half the number of chromosomes that the spermatogonium did. 

 These four spermatids all develop without further divisions into sper- 

 matozoa. We should now examine more closely as to what happens 

 during the reduction divisions. 



Maturation is a phenomenon common to both spermatogenesis and 

 oogenesis and is an essentially similar process in either event. Two 

 rapidly succeeding divisions, the reduction divisions, constitute the 

 important phase of maturation. These divisions effect a reduction 

 of the number of chromosomes by one half, and involve primarily a 

 quantitative equal, frequently combined with a qualitatively dissimilar 

 distribution of the fission products (chromosomes) among the resulting 

 cells. The actual numerical reduction of the chromosomes has already 

 occurred during synapsis when the chromosomes united into pairs, 

 forming bivalent chromosomes, or several may even have combined to 

 form plurivalent chromosomes. 



It is now believed that the pairs of chromosomes in synapsis are com- 

 posed of maternal and paternal elements and that their union repre- 

 sents the final stage in the fertilization process which resulted in the origin 

 and development of the organism whose germ cells are now in synapsis. 

 It is clearly known in several cases that the maternal and paternal chro- 

 mosomes do not fuse at fertilization nor during the several succeeding 

 segmentation divisions, and it is very probable that, in the germ cells 

 at least, the chromosomes from the two parents do not fuse until synapsis. 



Synapsis, as described in many insects and plants, usually takes 

 place during the telophase of the final oogonial or spermatogonial divi- 

 sion, though it has been observed to occur slightly earlier or later, some- 

 times even during the synizesis (contraction phase) of the chromatin 

 at the beginning of the growth-period of the oocyte or spermatocyte. 

 According to the observations of various investigators in the various 

 animal and plant groups, synapsis may be an end to end union of the ele- 

 ments of a pair of chromosomes or they may unite side by side. If they 

 unite in the former way, we have a case of telosynapsis; if by the latter 

 method, a case of parasynapsis. 



It is known on good evidence, in some cases, that one of the matura- 

 tion divisions separates entire chromosomes, and along the plane of 

 their previous union in synapsis. It is only reasonable to suppose that 

 the process is similar also in the more obscure cases. It is therefore 

 essential for a correct interpretation of the maturation phenomena and 

 the reduction divisions that we know how the chromosomes united in 

 synapsis. This is known in but few cases. A maturation division that 

 separates bivalent chromosomes into qualitatively dissimilar halves is 

 known as a reducing division; a division that separates chromosomes 



