The spermatogenesis of Hydra. 411 
spermary next to the mesogloea are often pale. As division occurs 
and some of the interstitial cells move out and become spermatogonia 
their nuclei stain very intensely, while the granules before mentioned 
decrease in number and gradually disappear. The conclusion seems 
to be that this nuclear nutrition material secreted by the endoderm 
nuclei and passed out to the ectoderm is absorbed by the rapidly 
dividing and growing nuclei of the primordial germ cells as the 
spermatogonia are formed. 
It will be noted that there is a marked contrast in color in 
the female and male of Hydra dioecia given in Fig. 6 and 7, — a 
contrast which has been found constant. The female shows the 
presence of much of the yellow-brown oily nutritive material, while 
the male shows very little. It will be remembered also that in the 
hermaphroditic forms such as H. fusca, the spermaries appear after 
the ovaries have been formed. The egg moreover is simply a cell 
containing a vast amount of nutritive material. It seems likely 
therefore that the yolk material which is used up in egg formation 
helps to produce that impoverished condition which seems invariably 
to accompany the spermatogenesis. This explanation would not 
account for the impoverished condition of the male of the unisexual 
species, but would lend itself readily to the hypothesis that when 
the season of sexual reproduction began these animals of H. dioecia 
which were fortuitously poorly nourished develope testes while 
those well nourished become females.  Disregarding theoretical 
considerations however, this much is established: That spermato- 
genesis is a process always accompanied by dearth of reserve nutri- 
tive material and that the necessary nutrition for the rapid cell 
multiplication of the interstitial cells is largely supplied by the 
endoderm, the nuclei of which cells elaborate it as a substance with 
the staining reaction of nucleinic acid. When passed to the ectoderm 
it lies among the interstitial cells, but decreases in amount as their 
nuclei increase in size and density of stain due to absorption of the 
nutrition. 
Mitosis occurs in what cells? 
In all the sections studied mitosis has been the universal mode 
of division in the interstitial cells, the exceptional mode in the ecto- 
derm cells and amitosis the constant rule in the endoderm cells 
Fig. 5 shows an ectoderm cell in process of amitotic division. At 
first it was thought this was the only mode of division of the ecto- 
Zool. Jahrb. XXI. Abt. f. Anat. 27 
