Further Studies on the Chromosomes III 



spermatocytes; the numbers are twenty-three and twenty-four, 

 and the odd chromosome can be identified among the twenty-four. 

 Stenopelmatus material, at best, is unfavorable for accurate count- 

 ing on account of the large number of chromosomes and the fact 

 that they rarely form flat plates. 



There seems at present to be no doubt that whenever an unpaired 

 heterochromosome is present in the first spermatocyte, the sperma- 

 togonial number is odd in the Coleoptera, Orthoptera and Hemip- 

 tera (cases with supernumeraries are an exception to the rule which 

 however, applies if the supernumaries are counted out). It is 

 equally certain that the unequal pair of the spermatocyte is also 

 found in the spermatogonia. In Tenebrio molitor and Photinus 

 pennsylvanicus the chromosomes of the somatic cells of the male 

 have been shown to be of the same number and character as those 

 of the spermatogonia. 



Not quite so certain is it that an equal pair of heterochromo- 

 somes in the female always corresponds to the unpaired one or 

 the unequal pair. This paper adds two more, Photinus penn- 

 sylvanicus and Photinus consanguineus, to the four species of 

 Coleoptera previously recorded ('05 and '06) as having such an 

 equal pair of female heterochromosomes. No exceptions have 

 been found, but it proves to be difficult to get suitable material 

 for determining the number of chromosomes in somatic cells. 

 The pupae would probably give good somatic mitoses in nearly 

 every case, but they are rarely to be had unless one can breed the 

 insects. In the flies there was no difficulty in finding dividing 

 oogonia and egg-follicle cells, and in every case an equal pair of 

 large heterochromosomes corresponded to the large and the small 

 one of the male ('08). In the Hemiptera an equal pair of hetero- 

 chromosomes has been determined in the female of a compara- 

 tively large number of species (Wilson '05, '06, '07, Stevens '06, 

 Boring '07). The consensus of evidence would therefore indi- 

 cate that this is the rule for these orders of insects, and that the 

 determination of sex is closely connected with fertilization, since 

 it is evident that only those eggs that are fertilized by spermatozoa 

 containing the odd chromosome or the larger of an unequal pair 

 of heterochromosomes can develop into females, and the males 



