OVOGENESIS OF HYDRA FUSCA. 65 



looks, therefore, as if the increase in size of the egg or eggs 

 might be the cause or at least the occasion of the multiplication 

 of these insterstitial cells. 



5. R. Hertwig ^ and others, maintain that the egg is merely an 

 interstitial cell which, after the ovary has begun to grow, increases 

 in size more rapidly than its fellows. The egg seems in my sec- 

 tions always recognizable as such in the adult hydra. It is 

 slightly larger than the inactive interstitials, has a very large nu- 

 cleus in proportion to the cell body, adjacent to which there lies 

 in this early stage a small dark ovoid body. The cell outline is 

 spherical, whereas the resting interstitials are polygonal in sec- 

 tion. There often appears at the early stage and always a little 

 later, as Kleinenberg pointed out, a vacuole near the nucleus. 

 All gradations from the large undoubted egg to this cell are 

 readily found. But intermediates between it and the interstitials 

 are, I may not yet say, wanting, but certainly rare. The evidence 

 seems to point then to distinct germ cells in the adult hydra. 



6. My results, furthermore, disagree with Kleinenberg's inter- 

 pretation of the nutrition of the egg and extend the observations 

 on the origin of the yolk granules as follows : At first the egg 

 is nourished as are the adjacent ectoderm and interstitial cells. 

 Material absorbed by the endoderm cells is massed in spherules 

 filled with brown droplets, perhaps granules. This is apparently 

 transformed, in large measure, in the endoderm cells and passed 

 to the ectoderm, as this brown material seldom appears in the 

 ectoderm cells. The endoderm cells elaborate also a material 

 which stains deeply with osmic acid. This is also passed to the 

 ectoderm, the cells of which are more heavily laden with it than 

 are the endoderm cells. It passes into the egg and interstitial 

 cells as well as others. The usual lecithin tests show it to be 

 that substance or a closely related one. The egg at first contains 

 the lecithin diffuse, but later in granular masses, " Pseudozellen." 

 The interstitial cells also absorb it and the nuclei become filled 

 with it, meantime enlarging considerably. Those interstitial cells 

 adjacent to the egg in the fairly mature ovary have their walls in 

 contact with the egg resorbed and the content of the cell becomes 



' " Uber Knospung und Geschlechtsentwicklung von f/. /tisca," Biol. Centralbl., 

 Vol. 26, 1906. 



