BIOLOGY OF PARACOPIDOSOMOPSIS. 



to one pole of the spindle. In this event one of the daughter 

 cells would contain no sex chromosome. Let us consider further 

 each of these possibilities. 



The mechanism of a disjunctional division was explained in 

 connection with the study of Copidosoma, from which a quota- 

 tion is given in the first part of this paper. It remains therefore 

 to point out what would happen should such a differential divi- 

 sion occur. If a disjunctional division should occur during 

 cleavage, the percentage of males to females would be deter- 

 mined by the time at which the differential division occurs. 

 For example, if disjunction took place at the first cleavage of 

 the fertilized egg, each of the two daughter cells would receive a 

 single X chromosome, and should no further disjunctions occur 

 in subsequent divisions, a pure male brood would be produced. 

 If on the same basis the disjunction did not occur until the 

 second division, only fifty per cent, of the brood would be males; 

 if at the third, twenty-five per cent.; if at the fourth, twelve and 

 one half per cent., and so on decreasingly until the point is 

 reached at which each cell becomes the progenitor of a single 

 embryo. However, in any random lot of mixed broods these 

 expected classes, based on per cent, of males would not be 

 realized, because there are certain factors in development which 

 would tend to modify the expected percentage of males in any 

 given brood. Thus not all of the cells divide simultaneously in 

 the early cleavages of the egg of these parasites. Some cells 

 may fail to develop altogether. More than one disjunctional 

 division may occur in the egg. Finally, some cells develop into 

 the so-called assexual larvae, which never reach maturity. In a 

 random lot of broods we should therefore expect to find a con- 

 tinuous series, such as is shown in the tables. 



Of the several possibilities suggested that of somatic non- 

 disjunction would seem to be the most plausible. First, because 

 it is in harmony with the known facts of cytology on cleavage 

 mitosis, in that all of the chromosomes are supposed to divide; 

 and second, because it is in accord with Bridges' ('16) work on 

 non-disjunction in Drosophila. This investigator has recently 

 suggested the possibility of somatic non-disjunction to account 

 for gynandromorphs and mosaics in a monembryonic egg, 



