The spermatogenesis of Hydra. 413 
the same primitive interstitial cells, and that he has observed all 
stages in the transition from interstitial cells to ganglion cell, nema- 
toblast and sex cell. My own observations are contradictory on this 
point, however. The characters of the sex cells as given above seem 
constant, and my conclusion would be that at some stage of the 
embryonic development certain cells are stamped with these charac- 
ters and that they and their progeny form the sex cells distinct 
throughout the life of the individual. Nor does it seem that 
SCHNEIDER’S statement without demonstration of the so-called inter- 
mediate stages ought to be accepted. Certainly the ‘intermediate 
stages’ between interstitial cells and the sex cells which he figures 
are nothing but stages of development of the spermatids from the 
spermatagonia. The latter and I believe the interstitials from 
which they are derived are clearly distinct from the other cell 
elements. 
Continuity of the germ plasm. 
I believe therefore that the sex cells are a distinct group of 
cells and that the germ plasm is then continuous in Hydra. But 
yet there are included in this interstitial layer cells of very diverse 
fates. It would certainly be a mixing of characters to designate 
that layer mesoderm from which are derived ganglion and sex cells. 
But evidently Hydra is on the border land between the animals 
with two layers and those with three. The higher protozoa un- 
doubtedly and some of the coelenterates probably have two germ 
layers. If the higher animals are derived phylogenetically from 
these we would expect to find some animals in which the meso- 
dermic characters were mixed. Then the interstitial layer of Hydra 
would represent mesoderm, in nascendi, with certain cells having 
distinct mesodermal] characters, others still bearing ectodermal char- 
acters. 
The mesoderm in nascendi. 
It was not intended however to revive discussion of this anti- 
quated germ layer theory, but merely to point out that the inter- 
stitial layer of Hydra might represent a transition phase from a 
two-layered condition to the three-layered of the higher animals. 
And even in those forms in which the three layers are distinct 
and well defined, it is known that they do not always retain their 
true function, but mesoderm may give rise to nerve cells, especially 
in regeneration. 
27* 
