The spermatogenesis of Hydra. 399 
Reduction in the number of chromosomes in marginal 
spermatogonia. 
In the marginal spermatogonia whose division approximates that 
of the interstitial or ectoderm cells as just described, reduction is 
foreshadowed as in those well within the spermary by the fusion of 
adjacent chromosomes as they converge toward the poles (Fig. 20). 
We then have six bivalent chromosomes, each containing four 
bivalent chromomeres. Diagrams 22—28 present a synopsis of this 
process. Fig. 22 gives a diagram of the equatorial ring of V-shaped 
chromosomes. These divide longitudinally beginning with the tip 
of the V (Fig. 23). Their open arms resist separation (24) and 
the chromosomes become rod-shaped while their four half chromo- 
meres fuse and form two. Thus what appears like a reduction 
division is not such at all (25). When separation of the daughter 
chromosomes does occur, each one contains two chromomeres (26). 
As the chromosomes converge toward the poles, adjacent ones fuse (27) 
to form the bivalent chromosomes of the spermatocytes. 
The recognition of this process came slowly. At first attention 
was confined to the spermary and the process of division as first 
outlined was worked out. Mitosis outside of the spermary was not 
encountered for a long time. Then there were found interstitial 
cells with the V-shaped chromosomes — cells of the size of the 
spermatogonia of the first generation with twelve V-shaped chromo- 
somes and most perplexing, cells of the size of spermatogonia of 
the first generation with apparently six V-shaped chromosomes seen 
in the late anaphase. When it was discovered that these latter 
chromosomes were made up of two rod-shaped chromosomes fusing 
and when cells like Fig. 19 showed how the V-shaped chromosomes 
became rod-like, the interpretation given was apparently the only 
one tenable. Other possibilities were considered however, and eli- 
minated; for instance that the germ cells were distinguished from 
the somatic interstitial cells by having only six chromosomes and 
that this number was doubled when the spermatogonia were formed; 
o again that only the germ cells showed the type of mitosis given 
in Fig. 36, while that of Fig. 21 was confined to somatic inter- 
stitial cells. But the facts cited disproved these and other similar 
hypotheses. 
