Studies on Chromosomes. 523 



mastes the number is 21 instead of 22 (Fig. 2, o, p) I confess 

 that surprise at this result was followed by skepticism regarding 

 all of the accounts asserting the occurrence of an even number in 

 other forms. This result, which was totally unexpected to me, 

 rests on the study of a large number of division-figures exactly in 

 the metaphase, many of which are of almost schematic clearness. 

 Of these, twenty-five (selected from six testes from different indi- 

 viduals, including both adults and larval forms) were drawn with 

 the camera, chromosome by chromosome, and subsequently 

 counted. Without one exception these drawings show exactly 

 twenty-one chromosomes; it is therefore out of the question that 

 my result (worked out on Paulmier's original preparations) can be 

 due to an accidental displacement of one of the chromosomes in 

 the process of sectioning, or to other similar sources of error. 

 I believe the error of previous observers on this point is owing to 

 the fact that one or more of the chromosomes sometimes show a 

 more or less obvious constriction near the middle, and the larger 

 ones are not infrequently curved sometimes almost into a 

 U-shape so that one might readily be mistaken for two. 



Quite in harmony with this result is the fact that in Anasa the 

 metaphase groups always show not two but three chromosomes 

 that are distinctly larger than the others, 1 one of these being 

 obviously without a mate of like size, while all the others may be 

 symmetrically paired, two by two, as a study of Fig. 2, o, p, will 

 show. It is obvious therefore that the heterotropic chromosome, 

 and hence the chromosome-nucleolus of the growth-period, must 

 be compared with one, not two, of the spermatogonial chromo- 

 somes. 



In Alydus the heterotropic chromosome appears in the con- 

 traction phase of the synaptic period as an ovoidal single body, 

 always attached to one side of a large plasmosome and imme- 

 diately distinguishable from the latter by its different staining- 

 reaction (Fig. I, a). Comparison of this figure with that of the 

 spermatogonial chromosomes (Fig. I, A), shows that the hetero- 

 tropic chromosome at this period is much larger than the two 

 spermatogonial microchromosomes united. In the spermato- 

 gonial equatorial plates of Alydus or Archimerus it is not possible 



'I regret to find myself here again in disagreement with Montgomery, who finds only two large 

 spermatogonial chromosomes in Anasa ('04, p. 151, Fig. 16). 



