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[B] different species belonging to 8[B] 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, though material was abundant, no metaphases could be found in which the chromosomes 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 Elateridæ, 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 exception has been found to the rule established by previous work on the Coleoptera (Stevens, '05) and on the Hemiptera (Wilson, '05 and '06), that (1) 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 spermatozoön (1) containing the small member of an unequal pair or (2) lacking one chromosome, must develop into a male, while an egg fertilized by a spermatozoön 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 chromosomes in the sense that they both represent sex characters and determine sex, one can not decide without further evidence.

Comparison of the two types in Coleoptera, especially where, as in the Carabidæ, both occur in one family, has suggested to me that here it is possible that the small chromosome represents not a degenerate female sex chromosome, as suggested by Wilson, but some character or characters which are correlated with the sex character in some species and not in others. Assuming this to be the case, a pair of small chromosomes might be subtracted from the unequal pair, leaving an odd chromosome. The two types would then be reduced to one. It may be possible to determine the validity of this suggestion for particular cases by observation or experiment.

Since the first of this series of papers was published, there have appeared three important papers by Prof. E. B. Wilson, bearing on the problem of sex determination in insects. These papers are based on a study of many species of the Hemiptera heteroptera. These insects fall into two classes—one in which a pair of "idiochromosomes," usually of different size, remain separate and divide quantitatively in the first spermatocyte, conjugate and then separate in the second maturation mitosis; and another class in which an odd chromosome—the "heterotropic" chromosome—divides in one of the maturation mitoses, but not in the other. Wilson regards the odd chromosome as the equivalent of the larger of the "idiochromosomes," its smaller mate having disappeared. In the somatic cells of the former class he finds in the male the unequal pair, in the female an equal pair, the smaller chromosome being replaced by an equivalent of the larger "idiochromosome." In the latter class the male somatic cells contain the odd number, the female somatic cells and oögonia an even number, the homologue of the odd chromosome of the male being present and giving to the female one more chromosome than are found in the male.

In his latest paper Wilson ('06) makes a variety of suggestions as to sex determination. He shows that if the "idiochromosomes" and the heterotropic chromosome be regarded as sex chromosomes in the double sense that they both bear sex characters and determine sex, the following scheme accounts for the observed facts in all cases where an unequal pair or an odd heterochromosome have been found:

Sperm. Egg.
{Large ♂ "idiochromosome"}
I.{or}+Large ♀ sex chromosome=a ♀
{Odd chromosome.}
II.{Small ♀ "idiochromosome"}
{or}+ Large ♂ sex chromosome=a ♂
{No sex chromosome}