of the process may go wrong under certain conditions, occasionally with 

 important cytogenetical consequences which will be discussed later. The 

 process as we have described it should be considered primarily as a 

 norm or base against which to view expected and observed variations. 



MEIOTIC MITOSES 



The transition from sporophytic to gametophytic tissue in plants and 

 the formation of gametes in animals is normally accomplished by a 

 special series of divisions known as meiotic mitoses or simply meiosis. 

 It may be defined as two divisions of the nucleus accompanied by only 

 one functional division of the chromosomes (Darlington. 1937). It 

 characteristically results in the separation of homologous chromosomes 

 and the halving of the chromosome number. The stages of the two divi- 

 sions and their more fundamental characteristics (Figures 5-6 and 5-7) 

 are as follows: 



First Division 

 Prophase I 



As in somatic mitosis, this is in general the stage of contraction. 

 There is, however, evidence that a slight elongation precedes the con- 

 traction (Belling, 1928; Wilson, 1939; Sparrow, et al., 1941). Meiotic 

 prophase is usually divided into a number of stages coinciding with the 

 occurrence of several phenomena normally associated with meiosis. 

 These stages and their characteristics are: 



Leptotene. The chromosomes are long, thin, optically single threads 

 which show little of the relic coiling characteristic of somatic prophase. 

 Presumably the coils unwind rapidly. In good preparations the threads 

 contain a series of chromatic beads called chromomeres which have 

 often been assumed to be representative of gene loci, though there is 

 no direct evidence on this point (Belling, 1928). 



Zygotene. Homologous chromosomes begin to pair, usually at the 

 ends or the kinetochore or both. General pairing is followed by a closer 

 chromomere-to-chromomere association or synapsis, which may be com- 

 plete or not, depending on the species in some cases and on conditions 

 within the organism in others. Many factors apart from homology are 

 known to influence synapsis; e.g., temperature, nutrition, and specific 

 genes. 



Pachytene. Contraction proceeds and synaptic mates tend to separate. 

 Also each chromosome can be seen to be double in some places, but 

 the separation is not as great as between homologues. In certain regions 



126 / CHAPTER 5 



