THE GENERAL OUTLINE OF MEIOSIS 49 



first place through a profound modification of two 

 mitotic divisions. It is very remarkable that in all 

 essential details meiosis is the same wherever it 

 occurs ; it is consequently possible to give a general 

 account of it which will apply equally well to the 

 gametogenesis of an insect and the sporogenesis of a 

 plant. 



The first meiotic division always has an elongated 

 prophase ; since this is in many ways different from 

 a mitotic prophase it is necessary to subdivide it 

 into a number of stages which, although they corres- 

 pond to the early, mid- and late prophase stages of 

 mitosis, have different names to indicate the main 

 processes which take place. The names of these 

 stages are, in order, leptotene, zygotene, pachytene, 

 diplotene and diakinesis. After diakinesis (which 

 corresponds to the end of prophase) comes a short 

 prometaphase, followed by the metaphase of the first 

 division (' First Metaphase '). 



In the following account we shall describe meiosis 

 in a diploid organism ; the meiosis of a polyploid is in 

 some respects more difficult to understand and is best 

 left until the details of the process in a diploid have 

 been explained. As no mention will be made of the 

 cytoplasmic phenomena of meiosis the description 

 will do for either spermatogenesis or oogenesis, 

 macro- or micro-sporogenesis, since there are no 

 constant differences between the nuclear phenomena 

 in the two sexes. 



Fig. 9. — Diagrams of the main stages of meiosis. Two pairs 

 of chromosomes AA' and BB' are shown, the A and A' 

 chromosomes having submedian spindle attachments, the 

 B and B' chromosomes having sub-terminal ones. Three 

 chiasmata are formed in the AA' bivalent, one in the BB' 

 bivalent. S.A. = spindle attachment, Ch. = chiasma. 

 There is no " terminalization " of chiasmata. In the 

 BB' bivalent a rotation of the arms takes place, in the 

 AA' bivalent there is no rotation. 



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