The spermatogenesis of Hydra. 393 
from interstitial cells and a very narrow zone immediately under 
the tentacles is also free from them. This agrees with observations 
of previous authors, yet I think that from the statements previous- 
ly made one would expect to find a much wider zone under the 
tentacles free from the interstitial cells than is found. They reach 
almost to the peristome and during multiplication in the formation 
of the spermary may be crowded up to this region (Fig. 7). 
Cell generations. 
The interstitial cell has a diameter of about 12 u. After repeat- 
ed divisions, necessarily repeated to form the large mass of the 
young spermary, there comes a time when the daughter cells formed 
do not grow to the size of the parent interstitial cell but remain of 
about half its volume. These cells are the first generation of 
spermatogonia. They divide to form the second generation of 
spermatogonia. During this division the chromosomes are reduced 
to half the somatic number and the spermatogonia of the second 
generation are transformed with little change to spermatocytes. 
The cell generations then from an interstitial cell or primordial 
germ cell to the sperm are: — 
(1) Primordial germ cell — an interstitial cell. From this by 
mitosis are formed (2) spermatogonia of the first generation. These 
divide mitotically and form (5a) spermatogonia of the second gene- 
ration, which are transformed without mitosis into (3b) spermato- 
cytes of the first order. Mitosis occurs forming (4) spermatocytes 
of the second order. These divide indirectly into (5) spermatids 
which transform to the spermatozoa. 
Spermatogonia. 
The spermatogonia of the first generation are at first nearly 
spherical cells about 10 « in diameter. Later as the spermary 
grows in size by the increase in the number of cells, they become 
dodecahedra or polyhedra with a flattening in at least one direction, 
frequently with three dimensions (Fig. 29). Calling the protruding 
tip of the spermary the pole, then the long diameter of the sperma- 
togonia will usually be meridional, the shorter diameter in the plane of 
a parallel and the shortest diameter radial. The dimensions being on 
an average 10 x 9% 7.5 u. But this is variable within narrow limits. 
The nucleus of the spermatogonia is about 7 « in diameter. It is usu- 
ally, possibly always, eccentrically placed, but whether the eccentri- 
city is always in the same direction I have not been able to deter- 
