102 bulletin: museum of comparative zoology. 



converted into a double thread, and at a much later period, after the 

 autosomes have become longitudinally split. We may, then, I believe, 

 look on the U-shaped monosome of the late growth and maturation 

 periods as derived by a longitudinal splitting of the flattened plate of 

 the earlier growth stages, the equivalent halves resulting from this 

 longitudinal splitting remaining connected with each other at one end 

 during the first maturation division, but becoming separated during 

 the second maturation division. This interpretation is borne out by 

 Stenobothrus, where the division is certainly longitudinal. 



B. Arphia tenebrosa. 



The maturation period in Arphia is practically the same as in Dis- 

 soteira. 



Figure 64 (Plate 4) shows the metaphase of the first maturation 

 division in a cell containing two monosomes. These elements are 

 plainly seen, one near each pole of the spindle. However, this con- 

 dition is not by any means constant, since in many cases both mono- 

 somes are near the same pole. Figures Pa~Pc (p. 100) show three 

 successive sections of the same cell during the early anaphase of the 

 first division, all the chromosomes being included. These sections 

 show conclusively that there are two monosomes, neither of which 

 divides during this division, and each is attached to mantle fibers from 

 only one pole. In this particular cell the monosomes are passing 

 to opposite poles of the spindle. Figure 5 is a polar view of tlie meta- 

 phase of the second division in a cell which evidently contains both 

 monosomes, since there are plainly thirteen chromosomes in the 

 equatorial plate. Both monosomes divide longitudinally during this 

 division, as does the single one in normal spermatocytes. 



It is obvious that in the case of the individual with two monosomes 

 the spermatids might contain eleven, twelve, or thirteen chromosomes. 



C. Hippiscus tuberculatus. 



The maturation period in this species differs little from the same 

 period in Dissosteira. 



D. Chortophaga viridifasciata. 



The maturation period in Chortophaga agrees in all essential 

 respects with that described for Dissosteira. 



