494 
Charles Edward Walker 
would at once be evident. The combination used was a Zeiss 2 mm 
apochromat lens with a Nr. 8 compensating ocular, of which the depth 
of focus giving sharp definition is not more than 0.7 /.i. The ehromosomes 
illustrated were drawn with an Abbe drawing apparatus, and none were 
drawn exeept tliose of which the whole outline eame sharply into focus 
at the same time; therefore the distortion from tilting cannot have excee- 
ded 0.7 /< in any case. When it is considered that in the cases of these 
ehromosomes the length is hardly ever less than 7 and in some as 
much as 18 //. it will be realised that the modifications in shape produced 
by differences in the angles of presentation can have been but verv shght. 
Moreover, many of the differences are such that no optical distortion 
due to the angle of presentation would have produced them, nor can 
thev all be due to differences in the stage of mitosis at which fixa- 
tion took place 1 ). In the case of each form illustrated, the chromo- 
somes were in cells, not onlv from the same testis, but from the series 
of sections on the same slide. The scale shown on the margin of the 
plate was arrived at by substituting a stage micrometer for the shde 
bearing the sections, after a chromosome had been drawn, and marking 
off the lines of the micrometer by means of the Abbe drawing apparatus, 
without altering the position of the board or anything eise. The par- 
ticular profile views of eacli chromosome were chosen as being those 
which would make any tilting least likelv to be admitted accidentally, 
and which would best show natural modifications in shape. It might 
possibly be supposed that in some instances a chromosome of another 
shape has been given as a modification of chromosome B in Triton . 
The only other chromosome occurring in this animal for which the 
profile view of B might be mistaken is shown in fig. 28a’. This chromo- 
some (fig. 28 x ) is figured by Moore and Arxold as shown in fig. 28 y, 
but I have found the former shape to be far more commonly approached 
though forms similar to the latter are to be found occasionally in the 
same slide. It will be seen that the difference in size makes any 
mistake extremelv improbable, quite apart from the differences in 
shape. Views of chromosome B which are not in profile are given in 
figs. 23 x and 23 y, but only to illustrate the shape of the whole. In the 
case of each form given, the first drawing is the typieal. the most sym- 
metrical or perfect, while the rest are treated as modifications of this 
shape. without any Suggestion being implied that one is more common 
than the other. It may be mentioned, however, that shapes similar 
*) Compare fig. 1 with 2, 3, 4, 19, and 21 and fig. 29 with 37, 41 and 42. 
