CHROMOSOMES OF PERLA ole 
The chromosome number in the second spermatocytic division 
is five. The five consist of two V’s, one rod, ‘m’, and X- or 
Y-chromosome (fig. 46). From what has been observed in the 
preceding division, it is to be expected that half the number 
of the second spermatocytes should contain one accessory chro- 
mosome, and the other half of them the other, and this is ap- 
parently what takes place here. 
All five chromosomes divide equationally in the second sper- 
matocytic division, neither of the accessories is heterotropic (figs. 
44 and 45). 
The anaphase of the division (figs. 48 and 49) is immediately 
followed by the formation of the spermatids (figs. 50 and 51). 
The further history of the spermatids has not been followed. 
GENERAL CONSIDERATIONS 
Pairing of chromosomes and probability of synapsis 
The idea of the paired association of chromosomes was first 
suggested by Sutton (’02), when he noted in Brachystola that 
all chromosomes could be associated into pairs according to the 
size characters. More data were accumulated later from both 
zoological and botanical sides, and there seems to be no doubt 
at present that, where chromosomes of different sizes and shapes 
are present, there are always two of each kind (excepting acces- 
sory chromosomes). In his extensive work on the topic, Metz 
(16) has shown with special clearness in about eighty species 
‘of Diptera which he examined, that the chromosomes are uni- 
formly associated in pairs in diploid cells, in all tissues, somatic 
as well as germinal, and in all stages of ontogeny (from egg to 
adult), and he stated that pairing chromosomes give an actual 
demonstration of a side-by-side approximation of corresponding 
chromosomes. 
The probability of synapsis becomes stronger, when inquiries 
are made as to the nature of the pairing. In a certain Hemip- 
teron, Wilson (’09) has described, beside the regular coupling 
of idiochromosomes of unequal sizes, that a small supernumerary 
chromosome which is indistinguishable from the m-chromosome 
