The Ovogenesis of Hydra. 311 



origin of the nutritive cells and the sex cells, we also find many 

 instances in which the egg cells are differentiated from the nutritive 

 cells early in the embryonic history and are impressed with their 

 distinctive characters. It is not surprising then, to find among the 

 hydroids an instance of such early differentiation. And this, of 

 course, is all that my contention amounts to; for, presumably, in 

 the embryology of hydra, the egg cells are derivatives of the inter- 

 stitials. Yet this is a matter for further study as is also the origin 

 of the distinctive germ cells in the bud. 



In a study of the origin of the male germ cells, I was led to 

 believe that the sex cells of the adult could be distinguished from 

 the interstitials (p. 413). The distinctive character of the germ cells 

 is more marked in the ovary than in the spermary; I therefore now 

 express my earlier tentative opinion with more certainty. 



It is evident tliat I can not agree at all with the recent de- 

 scription of the origin of the egg given by Tannreuther for H. 

 dioecia. He says that the interstitials multiply forming a mass 

 which separates into a central region that gives rise to the ovum 

 and a peripheral region, the temporary ovary. The central cells 

 enlarge, their walls break down while the nuclei enlarge and assume 

 the spireme condition. One nucleus, sometimes several, continue, 

 the others degenerate. The surviving one or ones form the egg or 

 eggs by again surrounding themselves with a cell body and wall. 

 So far as I can find, in my many specimens of H. dioecia, the cell 

 walls are always distinct; there is never evidence of degeneration 

 unless it be in animals exhausted by prolonged egg laying. The 

 spireme condition is only assumed during mitosis; it is prolonged 

 then. But other stages of mitosis of these same egg cells are common 

 even in eggs that have several times the volume of the interstitial 

 cells and that contain considerable yolk. 



14. The vacuole which appears near the nucleus, persists but 

 a short time in H. viridis but remains for a long time characteristic 

 of the egg in H. dioecia. 



15. Successive stages of the egg development are marked by 

 characteristic shapes. At first it is spherical or nearly so (Figs. 3 — 5, 

 Plate 11); this spherical phase is much more prolonged in H. dioecia 

 (Fig. 6, Plate 11), than it is in H. viridis; shortly after the egg becomes 

 conspicuous in the latter species, before it begins to ingest any 

 quantity of the surrounding nutritive material it is irregular and 

 amoeboid, but in H. dioecia the spherical shape is long retained and 



Zool. Jabib. XXVIII. Abt. f. Anat. 21 



