412 E. R. Downine, 
derm cells. But a few cases have been observed, only two or three 
in hundreds of sections, however, where mitosis occurred in an 
undoubted ectoderm cell. ScCHNEIDER had observed and figured 
mitosis in an ectoderm cell. But cell multiplication, except of the 
interstitials seems to be almost invariably amitotic. THazzwrrz (80) 
noticed that the nuclear division of the germ cells was indirect 
among hydroids in contrast to the other cells of the body. So con- 
stant is the indirect division among the germ cells and the direct 
among somatic cells that he feels safe in accepting mitosis as a 
distinguishing characteristic of the germ cells. Nussspaum (70) also 
notices that the sex cells are derived from the interstitial cells by 
mitotic division in Hydra. What the cause of this difference may 
be seems difficult to determine: whether it means an inherent differ- 
ence in the interstitial cells from the other body cells or whether 
the fact that they are crowded into the interstices of the other cells 
produces altered physiological conditions experiment alone can deter- 
mine. When living Hydra is teased up in water so that clusters of 
interstitial cells are set free, mitosis in them is still indirect. The 
cells do not live long enough to continue their division processes so 
that the condition of pressure anteceding the release may have deter- 
mined the style of this first division. 
The mesoderm. 
At any rate both structurally and physiologically this inter- 
stitial layer seems a distinct cell layer. Braver finds in the embryo- 
logy of Hydra that the endoderm and ectoderm are separated by a 
process of delamination. Some time after these layers have become 
distinct there is seen between the ectoderm and endoderm the inter- 
stitial cells — a third layer. This is formed from the ectoderm 
cells. Braver does not think this third layer should be considered 
mesoderm (p. 198). Certain it is that it has characters that are not 
mesodermal for from it are derived the nerve cells and the nemato- 
blasts as well as the sex cells. That the primitive interstitial cells 
do develop differently may be evidence that there are differences, 
though they are not histologically apparent. The recognizable 
nematoblasts are distinguished by some stage of the developing 
nematocyst; the ganglion cells by their smaller size, lack of granu- 
lation and processes; the sex cells by their very large nuclei, ex- 
tremely granular, and often by the presence of a Nebenkern (Fig. 15). 
SCHNEIDER says however that these three types are all derived from 
