Studies on Chromosomes. 403 



that as far as the Hemiptera are concerned neither the suggestion 

 I have made, nor the hypothesis of McClung has at present any 

 direct support in observed fact. 1 



The practical interest of the idiochromosomes lies in the very 

 definite basis that they give for an examination of the question by 

 the study of fertilization, for their disparity in size gives us the 

 hope of determining their history by direct observation. There 

 is good reason to believe that such a study will yield interesting 

 results. 



SUMMARY OF OBSERVATIONS. 



1. In Lygaeus turcicus, Coenus delius, Euschistus fissilis, 

 Euschistus sp., Brochymena, Nezara, Trichopepla and Podisus 

 spinosus all of the spermatids receive the same number of chromo- 

 somes (half the spermatogonial number), and no accessory chro- 

 mosome is present; but the spermatozoa nevertheless consist of 

 two groups, equal in number, which differ in respect to one of the 

 chromosomes, which may conveniently be called the "idiochromo- 

 some." 



2. In all of the forms named, excepting Nezara, half the 

 spermatozoa receive a larger, and half a smaller, idiochromosome. 

 In Nezara the idiochromosomes are of equal size, but agree in 

 behavior with the unequal forms. 



3. In all of the forms the idiochromosomes remain separate 

 and univalent in the first maturation-division, while the other 

 chromosomes are bivalent; this division accordingly shows one 

 more than half the spermatogonial number of chromosomes. 

 They divide separately in the first mitosis, but at the close of this 

 division their products conjugate to form a dyad, which in all the 

 forms save Nezara is asymmetrical. The number of separate 



'The discovery, referred to in a preceding foot-note, that the spermatogonial number in Anasa is 

 21 instead of 22, again goes far to set aside the difficulties here urged. Since this paper was sent to 

 press I have also learned that Dr. N. M. Stevens (by whose kind permission I am able to refer to her 

 results) has independently discovered in a beetle, Tenebrio, a pair of unequal chromosomes that are 

 somewhat similar to the idiochromosomes in Hemiptera and undergo a corresponding distribution to 

 the spermatozoa. She was able to determine, further, the significant fact that the small chromosome is 

 present in the somatic cells of the male only, while in those of the female it is represented by a 

 larger chromosome. These very interesting discoveries, now in course of publication, afford, I think, 

 a strong support to the suggestion made above; and when considered in connection with the com- 

 parison I have drawn between the idiochromosomes and the accessory show that McClung's hypo- 

 thesis may, in the end, prove to be well founded. 



