The spermatogenesis of Hydra. 391 
interstitial cells. They are much smaller than the ectoderm cells 
(see Fig. 12 and 15) and are characterized by a relatively large nu- 
cleus, which has a nucleolus. Furthermore this nucleus stains deeply 
and usually differently from the ectoderm nucleus. probably due to 
the fact that it is so active, dividing often, especially during sperma- 
togenesis. In some cases there appears near the nucleus of the 
interstitial cells a Nebenkern. It is from the interstitial cells that 
the spermatogonia are derived. 
Derivation of the spermatogonia. 
In a preliminary report (in: Science, V. 12, p. 228) it was stated 
that the spermatogonia were derived from the ectoderm. In very 
many cases the ectoderm cells along the margin of the spermary 
are seen to be dividing amitotically, which led to this conception. 
But careful measurement negatived such a conclusion, demonstrating 
that the interstitial cells were the immediate progenitors of the 
spermatogonia. The interstitial cells themselves may arise from the 
ectoderm cells. They do embryologically and it would not be sur- 
prising if they should so arise during spermatogenesis when they 
are needed in large numbers. Still, the ectoderm cells are usually 
too large to give the interstitial cells by a single equal division. 
Cells of a size intermediate between ectoderm cells and interstitials 
have very rarely been seen, and then doubtfully. The division of 
ectoderm cells has always been observed as equal or nearly so. It 
is not probable therefore that the interstitial cell is budded off from 
the large ectoderm cell. What evidence has been obtained would 
then tend to disprove the possible derivation of the interstitial cells 
during adult life, from the ectoderm cells. 
Early stage of spermary. 
In the preliminary steps of the formation of a spermary an 
interstitial cell divides mitotically (Fig. 12). The daughter cells 
grow to the parent’s size and then divide. Adjacent interstitial cells 
begin to divide, grow and multiply rapidly. They fill the space 
between the ectoderm cells and begin to distend the ectoderm, the 
cells of which elongate peripherally. The multiplication of inter- 
stitial cells increases at the initial point and the process spreads to 
a wider area. This increase in interstitial cells forms the early 
stage of the testis, — a conical elevation, covered externally with 
polygonal plates, the expanded outer ends of the ectoderm cells, the 
