The spermatogenesis of Hydra. 389 
what are those factors in the animals life history which cause the 
sexual organs to appear. It may be that one of the causes sug- 
vested above is the true cause in spite of the apparent negative 
results, and that it has not been allowed to operate under appro- 
priate conditions. Until experiments shall be found for Hydra that 
lead to the production of sexual reproduction at the will of the 
experimenter, it would be unwise to consider the experiments here 
recorded as sufficient to exclude from further consideration temp- 
erature changes, changes in ion concentration and osmotic pressure 
or varying conditions of food supply. Sufficient to say that thus 
far the results have been negative. 
Material and methods. 
The material for the histological study was collected as follows: 
H. dioecia, found sexually mature in park lagoons at Chicago, 
Oct. and Nov. A. fusca, collected sexually mature during Sept. and 
Oct. at Chicago in park lagoons, also at Beloit, Wis. For some of 
the material from the latter locality I am indebted to Mr. Grant 
SairH of Beloit. A. viridis, collected sexually mature, June and 
July at Chicago in park lagoons. H. grisea found in the park 
lagoons at Chicago but not in a sexually mature condition. 
Fixing agents. 
A variety of fixing agents have been used, including Osmic- 
MERKEL, HERMANNS, PERENYrS chromo-nitric, FLEMMINGS, GILSON’S 
mercuro-nitric, Carnoy’s acetic alcohol, KLEINENBERG’S picro-sulphuric, 
GRAFS chrom-oxalic, varying strengths of picro-acetic and hot corro- 
sive. The first three mentioned gave the best fixation, the osmic- 
MERKEL working especially well. A one-half per cent solution of osmic 
acid was used to kill the animals. The Hydra was placed in a watch 
crystal in as small a drop of water as possible and still allow the 
animal to expand well. When expanded about ten ccm of the osmic 
acid solution was quickly poured over it. In the majority of cases 
death is so instantaneous as to prevent contraction. The animal is 
left for a minute in the osmic and then transferred to MERKEL for 
twenty-four hours. It is afterward dehydrated with alcohol, using 
fifty per cent. first, cleared in cedar oil and imbedded in paraffin. 
Sections were cut 7 « in thickness usually, some 10 and some 3. 
Staining was done on the slide. 
