76 THE CHROMOSOMES 



first division, so that the half-univalents pass to the 

 poles although tardily. In the first three cases 

 (where the spindle attachments of the univalents 

 have not divided in the course of the first division) 

 the univalents behave normally at the second division, 

 separating into their component chromatid halves. 

 In the fourth case the spindle attachments have 

 divided and the univalents . separated into their 

 chromatid halves at the first division, and conse- 

 quently do not divide at all at the second meiotic 

 division, being passively distributed at random to 

 the two poles. 



Meiosis in the Male Drosophila and in other 



DrPTERA 



It is well known that in the males of Drosophila 

 spp. no crossing-over normally takes place (under 

 special conditions very rare cross-overs occur). ^^ The 

 reason for this has been cleared up by a recent 

 investigation. 32 j^q chiasmata are normally formed 

 in the autosomes ; if this were to take place in any 

 other organism but a Dipteran it would lead to the 

 bivalents separating into pairs of univalent chromo- 

 somes at diplotene, but owing to the strong ' somatic 

 pairing ' (residual attraction) the two chromosomes 

 of each bivalent do not fall apart, but remain 

 associated until anaphase. In the case of the X and 

 Y chromosomes it is usual for a pair of compensating 

 chiasmata to be formed in the neighbourhood of the 

 spindle attachment in a region which is inert and 

 ' homologous '. The normal process of meiosis in 

 the male Drosophila is thus highly modified, although 

 in the female it follows the usual course. 



In other Diptera such as species of the genus 

 Sciara even more unusual types of meiosis are found 

 in the males, although, as in Drosophila, oogenesis is 

 entirely normal. Thus in the male diploid set of 

 Sciara coprophila there are five pairs of chromosomes 

 (one pair of large V's, one pair of medium-sized 



