A Comparative Study of the Chromosomes etc. 
245 
parations, which they w^ere the first to employ) has been answered by 
Lefevre and McGill (’08), whose work agrees with that of Wilson. 
Tliat smear preparations often present very misleading appearances, I 
shall try to show in this paper. Henning (’91), Paulsuer (’99), Gross 
(’04a, ’04b, ’06), Wilke (’07), and Montgomery (’ll) have dealt in 
soine detail with the formation and coinposition of the tetrads in sper- 
matogenesis oi various Hemiptera, and with the deportment of the tetrads 
in the matiiration divisions. With the revival and increased study of 
the qiiestion of the syndesis, and the evolution of the gemini found in the 
first sperniatocyte, much has been done in the past ten years to give us 
a better idea of the early and later stages of the growth period. Hen- 
king’s conclusions with regard to the coinposition of the tetrads is con- 
firnied by more recent work. This is also true of Pauliher’s conclusions, 
although the method of conjugation by metasyndesis may be questioned. 
Gross’ interpretation of the formation of the tetrads, by the lengthening 
of the short arms of cross tetrads of early prophase of the first maturation 
division, has not been substantiated by further investigation. Nor have 
we any case parallel to that of Wilke (’07), who describes the same 
number of chromosomes in the spermatogonia and spermatocytes of 
Hydronietra, and concludes that the tetrads are univalent, being made 
up by the end-to-end conjugation of half chromosomes. This results 
in prereduction and synmixis by metasyndesis of unlike halves. In 
Hygrotrechus, a near relative of Hydronietra, Montgomery (’06) has found 
the spermatogonial number (21) and reduced number (11) to be in accord 
with that of other Hemiptera possessing an unpaked “N-chromosome”. 
Browne (’IO), in a paper entitled “The Relation Between Chromo- 
some-Number and Species in Notonecta", gives a very interesting com- 
parison of the chromosomes in several species of these closely related 
insects. The diploid number of chromosomes in Noctonecta undulata is 
26, in N. irrorata 24. This reduction in number in irrorata comes about 
by the tendency of a small autosome pair to fuse with a larger autosome 
pair. N. insulata represents transitional conditions (between N. undulata 
and N. irrorata) in this fusion, sometimes exhibiting 24 and sometimes 
26 chromosomes. There is however, another possibility, namely that the 
smaller number of chromosomes (N. irrorata 24) is the more primitive 
condition and that the larger number (N. undulata 26) has come about 
by a tendency of the large autosome pair to separate into two autosome 
pairs. Wilson (’lla) has also shown that in Nezara there exists a 
compound chromosome, probably made up of a small and large pair 
fused together laterally in the form of a butterfly. 
