1895.] Notes on the Reproduction of Plumularian Hydroids. 971 
Stained sections of these hooks failed to throw much addi- 
tional light on the subject, the only noticeable histological 
feature being an appearance of great activity in cell multipli- 
cation, and the presence of an unusual number of nematocysts. 
These sections were of value, however, in demonstrating that 
the enlargement of the stem termination was not due to the 
presence of a parasite, as is sometimes the case among hydroids, 
e. g., Syncoryne eximia and Coryne mirabilis. 
The clasping of the hooks is probably effected mechanically 
by the undulations of the ripples passing along the alga which 
supports the hydroid colonies. 
Conjugation is essentially the union of two individuals of a 
species during which an interchange of protoplasm is effected 
without the intervention of ova or spermatozoa. So far as I 
have been able to discover this process has not heretofore been 
found among the metazoa, and the observations recorded above 
must be regarded as merely an indicatior. of the possibility of 
conjugation among hydroids. 
It is now a wellestablished fact that the sex cells, both male 
and female, of the Plumulariide originate in the endoderm of 
the stem ; and any process which would enable the contents of 
the endodermal cells of one stem to mix with the contents of 
the endodermal cells of the stem of another colony would ren- 
der conjugation possible so far as the purely mechanical part 
of the question is concerned. This would be effected in the 
case under consideration by the solution of the contiguous 
walls of the hooks when clasped as already described. While 
this solution was not actually seen in any of the specimens 
described by me, it was found that the perisare was usually 
thinner in the region of contact than elsewhere. 
It must be remembered, moreover, that in the normal repro- 
duction of most hydroids a solution of the perisarc of the stem 
is effected, probably by chemical action, whenever a gonan- 
gium is formed,‘ and therefore no new principle would have to 
3 The permanent union of individuals which results in Diplozoon cannot be 
termed conjugation in the sense eet used, because in the Diplozoon the interyen- 
tion of ova and sperma 
t+“ Die Entstehung der Sexualzellen bei den Hydromedusen.”” Dr. August 
Weismann, p. 182. 
