44 CYTOLOGY CHAP. 



In many cases meiosis in one and the same species has been interpreted 

 by one worker as parasyndesis and by another as telosyndesis. The 

 point of difference between the two schemes lies in the observation and 

 interpretation of the zygotene, pachytene and diplotene stages. 



Syndesis as described above in Tomopteris, Lepidosiren and certain 

 insects, and illustrated diagrammatically by Fig. 15, is parasyndesis, so 

 called because in the zygotene nucleus the homologous chromosomes lie 

 side by side and conjugate along their lengths. This interpretation of 

 the zygotene stage, with its consequent reading of the pachytene and 

 diplotene stages, was first given in its present form by von Winiwarter 

 (1901), and has since gained wide acceptance. To the Louvain school of 

 cytology is specially due the credit of having established the hypothesis 

 on a firm basis, both by new observation and the review of old work in 

 the new light (Gregoire, 1910). 



The theory now known as telosyndesis was first proposed by Mont- 

 gomery in 1903 for various Amphibia, and independently by Farmer and 

 Moore in 1905 for several other forms. It must, however, be mentioned 

 that Montgomery subsequently (1911) gave up his original view in favour 

 of the theory of parasyndesis. 



According to the theory of telosyndesis the thin threads which are 

 seen joining together in pairs in the zygotene stage are not whole chromo- 

 somes conjugating to form a bivalent, but are the temporarily separated 

 daughter halves of chromosomes split for forthcoming mitosis. On this 

 view, therefore, the duplicity of the chromosomes in Figs. 14, D, E, F, and 

 1 6, F, is of precisely the same nature as the duplicity of somatic prophase 

 chromosomes (cf. Figs. 3, 7, 8, etc.). The undisputed fact that the 

 number of thick bands in the pachytene nucleus is haploid is supposed to 

 be due to the fact that each consists of two homologous chromosomes 

 joined end to end, as shown in the diagrammatic figure. 



So far we have been dealing with what is mainly a matter of interpre- 

 tation and not of observation, but the next stage in the process involves 

 a difference of opinion on a matter more nearly approximating direct 

 observation. The telosyndetic view requires that the double chromo- 

 somes of the diplotene stage are not formed, as described above, by the 

 reopening of the space between two approximating chromosome threads 

 of the zygotene nucleus, but by the approximation of the limbs of the 

 horse-shoe shaped pachytene bands. They may then join at the free 

 ends to form rings instead of U's, or break across at the point of junction 

 to form two separate but adjacent chromosomes, together constituting 

 a bivalent. 



The constitution of the bivalent s is now the same in both schemes 

 (Figs. 15, E, and 18, C'), and the further course of meiosis is described alike 

 in both cases. In Fig. 18 are shown three stages in the formation of the 



