108 PHYSICAL BASIS OF HEEEDITY 



opening out in various ways to produce figures like those 

 shown in Fig. 47, c. 



The sex-chromosome {X) that has no mate in the 

 Plirynotettix male, and hence has not conjugated, has only 

 one longitudinal split (a dyad). The cell, the primary 

 spermatocyte, with its nucleus next divides. Eleven auto- 

 somes go to each pole, and the sex-chromosome failing 

 to divide at this time goes to one daughter cell only. The 

 secondary spermatocytes are produced — half with 12, half 

 with 11 double chromosomes. A short resting stage follows 

 — the chromosomes again becoming diffuse, ie., forming 

 vesicles. They soon reappear and a second division takes 

 place, producing the spermatids — the daughter cells of 

 the secondary spermatocytes. Half of these have 12, half 

 11 chromosomes — the X-chromosome having divided at 

 the second division. 



Wenrich found it possible to identify certain of the 

 chromosomes and was thus enabled to follow a few of 

 them through several successive stages. Eight consecu- 

 tive stages in the history of chromosome ^'B^' of Pliryno- 

 tettix are shown in Fig. 48. Indications of the primary 

 split are present in a, h, c, the secondary split appears 

 first in d. The evolution of the thread continues as the 

 tetrad becomes placed in the spindle in such a way that 

 the first separation of the chromosomes takes place along 

 the secondary split, i.e., the first division is equational. 

 Wenrich found in several other individuals of this species 

 that this same chromosome pair ^'B^^ consist of unequal 

 members as shown in Figures 48, 2 Or-h and 3 a-d. In 

 48, 2 (7 a distinct crossing of the threads is present. The 

 shape of the contracted chromosome (f g h) and its posi- 

 tion on the spindle show that one of the longer, and one 

 of the shorter strands passes to one pole, and similarly a 

 longer and shorter to the other pole. The division here is 

 in the plane of the secondary split, i.e., equational. The 

 inequality in length of the conjugating pair makes this 

 conclusion certain in this case. 



