OF THE OERM CELLS OF METAZOA. 



199 



phases, up to the monaster stage of the first mitosis, the longitudinal split generally closes 

 temporarily. The definitive form of these chromosomes in all the Hemiptera examined 

 is that of a dumbbell which may be either straight or bout ; ring forms are more infre- 

 quent, but are occasionally found in all species. In the Hemiptera, as in Peripatus, 

 each ring may be conceived as a dumbbell which has become bent until its ends meet, 

 and accordingly the hollow of the ring is not the longitudinal split, but a space sepa- 

 rating the univalent chromosomes. That is to say, genei'ally only one end of one 

 univalent chromosome is joined to one end of the other, but in the ring form both ends 

 of the one are joined with both ends of the other. In all bivalent chromosomes that 

 have not, the ring form a longitudinal axis can be plainly determined, and generally each 

 univalent chromosome is elongated in the same line; the constriction perpendicular to 

 this long axis is a true transverse division, and is the band of linin joining the ends of 

 the two univalent chromosomes. This orientation of the axes of the bivalent chromo- 

 somes allows the positive determination of the manner in which the chromosomes are 

 halved in the maturation divisions. 



In the first maturation division in all the Hemiptera examined the bivalent chromo- 

 somes are transversely divided, /'. e., whole univalent chromosomes arc separated ; in the 

 anaphase of this division the longitudinal split of the univalent chromosomes reappears, 

 and in the second maturation mitosis the univalent chromosomes are halved through the 

 plane of this split (equation division). The valences of the chromosomes in the successive 

 generations are accordingly: spermatogonium, univalent; first spermatocyte, bivalent; 

 second spermatocyte, univalent; spermatid, semivalent. The classing of the chromosomes 

 as semivalent in the spermatid may appear surprising, for they have always been consid- 

 ered univalent; but they must be considered semivalent with reference to the number in 

 (he spermatogonia in those cases, as in all the Hemiptera examined, where the second 

 maturation division follows immediately upon the first without any indication of an inter- 

 mediate rest stage. 



Thus in the Hemiptera, as in Peripatus, the maturation divisions do not accomplish 

 the reduction in number of the chromosomes, for this takes place long before in the 

 growth period; the first maturation division separates entire univalent chromosomes 

 (pseudoreduction, Ruckert, 1894), the second halves each univalent chromosome equation- 

 ally, and thus halves the chromatin mass. Though the chromosomes of the spermatid are 

 logically semivalent with reference to those in the spermatogonia, yet they are potentially 

 univalent on account of the increase in mass of the chromatin during the growth period 

 (where at least a doubling of the mass occurs). 



In my study on Peripatus (1901) it was shown that the individuality of the chromo- 

 somes is maintained from the last spermatogonic mitosis up to and through the maturation 

 A. P. S. — VOL. XX. Z. 



