MEIOSIS 



115 



postmeiotic division. Cytokinesis follows in typical cases, giving a 

 quartet of uninucleate cells (spores; spermatids soon to become sper- 

 matozoa; egg and polocytes). 



Problems of Meiosis. — The foregoing account of meiosis will serve as a 

 l)rercquisite to our later discussion of the mechanism of M(?ndelian hered- 

 ity, but it does not full.y indicate the prominent position that the ]:)rocess 

 holds in present-da}^ cytological research. Meiosis, like practically (»very 

 other process in the organism, continues to present numerous problems, 

 the solution of which would shed needed light upon phenomena occurring 

 at various stages of the life cycle. We should 

 therefore consider for a moment a few such 

 questions now engaging the special attention of 

 cytologists. 



What initiates meiosis? What are the physi- 

 ological conditions associated with the change 

 from the ordinary somatic type of nuclear divi- 

 sion to the meiotic type, and what relation do 

 these conditions bear to the prior changes that 

 bring on the reproductive phase in the organism? 

 Experimental treatments frequently result in the 

 partial or complete replacement of the meiotic 

 b}^ the somatic type of division in meiocytes, and 

 various suggestions have been made, for example, 

 regarding the apparent effects of a retardation 

 or an acceleration of the prophasic process upon 

 the character of the division. So far, however, 

 the main question has received no satisfactory^ 

 answer. 



What causes synapsis? The synaptic reac- 

 tion occurs between apparently single chro- 

 mosomes having the peculiar constitutional 

 relationship designated as homology, and only 



under physiological conditions that are essentially normal. It is 

 manifested primarily between corresponding minute portions or 

 units in the two threads, synapsis being normal in all respects 

 only when the various units or portions in the two have the same 

 serial order. Moreover, if three or more homologous chromosomes 

 are present in the meiocyte, only two as a rule synapse closely at any given 

 region, as though the synaptic force were somehow neutralized by the 

 union of two homologous portions (Fig. 86). In the "somatic synapsis" 

 of salivary-gland chromosomes three or more do unite closely. These 

 phenomena remind one of electrical attractions, the agglutination of 

 bacteria, the formation of one-strain groui^s of myxobacteria at the time 



Fig. 80. — Trivalent 

 chromosome in meiotic pro- 

 phase in maize. Each 

 member shows its two 

 chromatids in regions not 

 in synapsis. The kineto- 

 chores of all three members 

 lie in contact. {From a 

 preparation by B. Mc- 

 Clintock.) 



