THE GENERAL OUTLINE OF MEIOSIS 61 



Leptotene 



This is the earliest part of the prophase of the first 

 meiotic division — it corresponds to the very beginning 

 of prophase in an ordinary mitosis . The chromosomes 

 in the leptotene stage resemble those of the early 

 prophase of mitosis except for one important 

 feature ; they are not longitudinally divided — in other 

 words each consists of a single chromatid and not of 

 two chromatids held together throughout their length 

 as in the case of mitosis. Another point of difference 

 which is somewhat variable is that the leptotene 

 chromosomes are rather clearly made up of a series 

 of granules (called chromomeres) connected by non- 

 staining intervals ; this may also be the case at 

 mitosis, but it is not usually so obvious. As in the 

 case of the granules in the salivary gland chromo- 

 somes it has naturally been suggested that the 

 chromomeres are actual genes. In various Liliaceae 

 the total number of chromomeres in the whole 

 chromosome set has been counted and found to lie 

 between 1,500 and 2,500 ^ ; of course we do not know 

 the total number of genes in these plants, but in 

 Drosophila the total number (including the hitherto 

 undiscovered ones) has been estimated at 14,000.^^ 



The leptotene chromosomes are present in the 

 same number as in the somatic tissues ; very often 

 they are not arranged at random inside the nucleus, 

 but preserve the arrangement of the previous tele- 

 phase (with all the spindle attachments together at 

 one side of the nucleus and the chromosomes arranged 

 as in a bunch of flowers ^^ ; in this case they are 

 said to be polarized. 



Zygotene 



Ijcptotene is usually a stage of short duration. It 

 is followed by a stage called zygotene in which the 

 homologous chromosomes come together in pairs and 

 become closely approximated throughout their length. 



