Studies 072 Chromosomes. 373 



this point by failing in some cases to distinguish between the two 

 divisions. 



On tracing out the history of the two divisions step by step, 

 decisive proof was obtained that the apparent reduction in number 

 is brought about in the period immediately following the final 

 anaphase of the first division (which coincides with the earliest 

 prophase of the second division) by a conjugation of two unequal 

 chromosomes that occupy the center of the equatorial plate in the 

 first division and evidently correspond to some of the forms 

 designated by Montgomery as "chromatin-nucleoli." This pro- 

 cess can be determined with certainty, owning to the fact that in all 

 of the species, with a single exception, one of the two conjugating 

 chromosomes is much smaller than the others, while in Lygaeus 

 both are much smaller, and they are very unequal in size. The 

 central dyad of the second division is therefore asymmetrical, one 

 of its constituents being in Lygaeus not less than five or six, and 

 in Coenus not less than two or three, times the bulk of the other. 

 The two unequal constituents of this dyad are then immediately 

 separated again in the ensuing division in such a manner that tn 

 both species one half the spermatids receive the smaller^ one half 

 the larger-^ moiety of the central chromosome (or dyad) of the second 

 division. ^An essentially similar process was ultimately found to 

 occur in^uschistus fissilis, in another undetermined species of 

 the same genus, in Brochymena, Nezara, Podisus and Tricho- 

 pepla. The first four of these show the same chromosome- 

 numbers as in Lygaeus and Coenus. In Podisus the number is in 

 each division one more than in the corresponding divisions of the 

 other genera (z. e., respectively 9 and 8 instead of 8 and 7, while 

 the spermatogonial number is 16 instead of 14).^ Nezara differs 

 from the other genera in the fact, which is of importance for a 

 comparison with such forms as Anasa or Alydus, that the two 

 chromosomes which undergo conjugation after the first division 

 are of equal size; so that in this form the two classes of spermatids 

 are indistinguishable by the eye. Since the eight species I have 



^In several of these cases the numbers do not agree with those given by Montgomery ('oi, i). I 

 believe this observer to have been misled by the fact, which he also observed in some cases, that the first 

 division shows one more than half the spermatogonial number of chromosomes; and it is easy to mistake 

 the latter number owing to the fact that the larger spermatogonial chromosomes often show a more or 

 less marked constriction in the middle. Slightly oblique views of the late metaphase, when the chromo- 

 somes are double, may also readily give an erroneous result. 



