486 
Katharine Foot and E. C. Strobell 
These two species of Hetniptera offer another favorable fcature — 
büth have only 14 diploid chromosomes and this limited number makes 
it quite impossible to confuse the X Y chromosomes with the so-called 
ordinary chromosomes. 
Methods. Both sections and smears were used for the parent species 
{E. variolarius and servus)\ biit the smear method only ^Yas used for the 
hybrids. A comparison of smears with sections shows, in all cases, that 
the form of the chromosomes is demonstrated more clearly in smears 
than in sections and the smears have the added advantage of showing 
every chromosome in the group and each in its entirety. 
Those tvho conclude that smears give imrehable results cannot have 
used the method properly. We never smear an entire testis on one slide 
iinless this is necessitated by scarcity of material, for if too much material 
is used it is impossible to isolate the cells properly. We place a testis on 
one end of the süde and with a fine needle (Xo. 14) cut front it the area 
containing the cells at the stage required, the location of the cells having 
beeil first ascertained by the examination of sections. The selected area is 
dissected from the testis in a th'op of water and these cells are then pushed 
by the point of the needle to the centre of the slide. The cells are then 
gently tapped apart by the point of the needle — never roughly spread 
over the shde by the length of the needle as is the usual method in sniea- 
ring. The aini is to separate whole cells front each other and allow theiii 
to flatten and th’y in a single layer, exactly as we isolate germinal vesicles 
front the growing eggs by pricking each egg and allowing the cytoplasm 
to flow out, carrying the germinal vesicle with it. 
In smearing the testes, we have found it an advantage — when the 
chromosomes only are needed — to acidulate, with acetic acid, the (h'op 
or two of water in whicli the cells are tapped apart. This disintegrates 
somewhat the cytoplasm; but does not disturb the chromosomes, wliicli 
are cpiite as perfect as the chromosomes of germinal vesicles for which 
we use only distiUed water, have found acidulated water offers no 
advantages for any of the ovarian stages; but a trace or more of acetic 
acid in the water used for the testes and embryonic cells acts like a 
gentle disintegrating agent and aids in separating the cells with as 
little disturbance as possible. We have found it advisable to use both 
methods (with and without acid) for in sonie stages one or the other 
may give the best results. We have found the use of acetic acid a disad- 
vantage for the ovaries we have studied, for it behaves as a fixative 
for these cells and this coagulation makes it more difficult to separate 
the cells properly. For the testes, on the contrary, even 5% or 
