GENERAL DISCUSSION. 53 



GENERAL DISCUSSION. 



It will be seen from the foregoing that the results obtained in the 

 Study of the germ cells of Tenebrio molitor have been confirmed in full 

 for several species of Coleoptera, and in part for 19* different species 

 belonging to 8* families. It has also been shown that a different type 

 of Coleopteran spermatogenesis exists in at least 3 families, where an 

 odd chromosome like that in the Orthoptera occurs in place of the 

 unequal pair. In all of these insects the spermatozoa are distinctly 

 dimorphic, forming two equal classes, one of which either contains 

 one smaller chromosome or lacks one chromosome. 



The most difficult part of the work has been the determination of 

 the somatic number of chromosomes in the male and female. In some 

 cases suitable material has been lacking ; in others, thoiigh material 

 was abundant, no metaphases could be found in which the chromo- 

 somes were sufficiently separated to be counted with certainty. In 

 three species (in addition to Tenebrio molitor) where the unequal pair 

 is present, the female somatic cells have been shown to contain the 

 same number of chromosomes as the spermatogonia, but an equal 

 pair in place of the unequal pair of the male. In two new cases the 

 male somatic number and size have been shown to be the same as in 

 the spermatogonia . In one of the Elateridse , where the spermatogonial 

 number is 19, the female somatic number is 20, and in Aphrophora the 

 numbers in male and female cells are respectively 23 and 24. No ex- 

 ception has been found to the rule established by previous work on 

 the Coleoptera (Stevens, '05) and on the Hemiptera (Wilson, '05 and 

 '06), that (i) in cases where an unequal pair is present in the male germ 

 cells, it is also present in the male somatic cells, but is replaced in 

 the female by an equal pair, each component being equal in volume 

 to the larger member of the unequal pair, and (2) in cases where an 

 odd chromosome occurs in the male, a pair of equal size are found in 

 the female. It is therefore evident that an egg fertilized by a sper- 

 matozoon (i) containing the small member of an unequal pair or 

 (2) lacking one chromosome, must develop into a male, while an 

 ^g% fertilized by a spermatozoon containing the larger element of an 

 unequal pair of heterochromosomes or the odd chromosome must 

 produce a female. 



Whether these heterochromosomes are to be regarded as sex chro- 

 mosomes in the sense that they both represent sex characters and 

 determine sex, one can not decide without further evidence. 



*AuG. 20, igo6. — 36 species belonging to 12 families. See note, p. 49. 



