PROXIMAL PRECOCITY 



309 



represented simply as a timing difference at meiosis, the proximal 

 parts being in advance of the distal parts. Moreover, since it 

 applies to all chromosomes regularly in exactly corresponding 

 regions it can scarcely be due to a differentiation of the genetic 

 properties of the chromomeres ; it must be due to the position of 

 the precocious parts in relation to the centromere. At the same 

 time we must notice that its location is strictly comparable with 

 that of prochromosomes in plants with small nuclei [cf. Ch. XII). 

 In Mecostethus a localised precocity of chromosomes is found 



/>. melancgaster 



D. tuiubria 



D. virilia 





Fig. ioi. — Diagram showing the parts of the chromosomes of three 

 Drosophila species which are differentially condensed in pro- 

 phase and, below, for comparison, their resting nuclei with dark 

 prochromosomes. (After Heitz, 1933 b.) 



at meiosis, but it is the distal parts of the chromosomes that are 

 affected (Janssens, 1924 ; McClung, 1928 h ; White, 1936). 



(C) The third type of differential behaviour, like the second, 

 is peculiar to meiosis or to cells that are about to undergo meiosis. 

 Like the first, on the other hand, it is characteristic of particular 

 parts of some of the chromosomes, and the parts may be proximal 

 or distal or include the whole chromosome. This behaviour is the 

 well-known " heteropycnosis " or differential condensation of the 

 sex chromosomes in animals. 



In order to understand the cause of this behaviour it is necessary 

 to consider all cases of the kind together. These show that the 



