11 MEIOSIS IN INSECTS 



41 



Here again the joint takes the form of a sharp angle in the longer 

 chromosomes and of a deep constriction in the smaller ones. Evidently 

 the form it shall take depends upon the relative length and breadtli 

 which a given chromosome has attained at a given moment. 



As commonly happens in meiosis, there is usually no resting stage 

 between the two meiotic divisions in Lepidosiren. As soon as the chromo- 

 somes approach the poles in anaphase I. new spindles are formed and the 

 chromosomes, now longitudinally split, become arranged in the meta- 

 phase equatorial plates. Since each chromosome is still transversely 

 constricted as in the first division, figures very similar in appearance to 

 those of metaphase I. are obtained, although of course each tetrapartite 

 chromosome is now constituted out of two transversely constricted 

 daughter chromosomes, instead of out of two juxtaposed transversely 

 constricted whole chromosomes. 



The pair of long chromosomes already alluded to should be noted in 

 Fig. i6rt, K, L, M. From the study of favourable nuclei, it can be estab- 

 lished that these are formed by the breaking apart of one of the rings of 

 the diplotene stage. That is to say, they conjugated to form one bivalent 

 in syndesis, an example of the evidence mentioned above that conjugation 

 is not haphazard, but takes place between homologous chromosomes. 



(3) Meiosis in Certain Insects (Fig. 17) 



The Insects are a group which have attracted much attention from 

 cytologists, the most important recent work on their meiosis being that 

 of Wilson (1912) on certain Hemiptera (Fig. 17). 



The spermatogonial telophase (Fig. 17, A) passes into a confused 

 network (B) in which no chromosome limits are visible. The beginning 

 of the meiotic phase is marked by the appearance of a number of massive 

 chromatin bodies, of the diploid number (C). (The two denser bodies 

 visible at this stage, and more conspicuous in D-J, are the sex chromo- 

 somes, to be described in Chapter IV.) 



The leptotene nucleus is derived from this stage by the resolution of 

 each one of the chromatin bodies into a spirally coiled hlament, wliich 

 spreads out and interlaces with the other similarly formed filaments to 

 produce the leptotene nucleus. This is followed by synizesis, and this 

 by the pachytene stage. The mode of syndesis cannot be traced, but 

 since the leptotene threads are in diploid number and the pachytene 

 threads haploid, and since each pachytene thread spHts longitudinally 

 into two in the diplotene stage (I), it is to be assumed that syndesis of 

 the type described for Tomopteris and Lepidosiren took place during 

 synizesis. 



The fact is notable that neither the leptotene nor pachytene threads 

 are orientated to form a bouquet as they are in Tomopteris and Lepidosiren. 



