302 Elliot E. Downing, 



probable that the eggs develop upon fertilization only and not partheno- 

 genetically as Tkembley and other of the early investigators thought 

 they developed. ECKEE, publishing in 1853, maintained that the whole 

 body of hydra consists of a homogenious substance, partly clear, partly 

 granular, a substance both elastic and contractile : CoEDA, Baumgaetnee 

 and others, notably Leydig, demonstrated the cellular character of hydra 

 and that the cells were arranged in distinct layers. Leydig not only 

 saw the egg but its nucleus, also ; he unconsciously discovered, too, the 

 interstitial cells, which he plainly figures but does not recognize as distinct 

 from the more conspicuous ectoderm cells. It remained for Kleixexbeeg 

 to clearly recognize the interstitial layer and its importance. „Das 

 Ectoderm besteht aus zwei Geweben, aus dem Neuromuskelgewebe und 

 dem interstitiellen Gewebe : das letzte bildet in seinen Zellen die Nessel- 

 kapselu und aus ihm gehen die Geschlechtsoi'gane hervor." Hydra, p. 27. 

 It was manifestly impossible, until these discoveries were made, to 

 describe even the external happenings in the formation of the egg and 

 ovary with any greater degree of accuracy than had the very early in- 

 vestigators who recorded the appearance of a diminutive mound-like 

 swelling out of which the spherical egg came. These external events 

 Kleenenbeeg describes with remarkable clearness ; his descriptions of 

 the histological details are no less accurate, as a rule, as far as they go. 

 I shall, therefore, give a description of the ovogenesis in a free translation 

 of his own words, leaving out much that is, for our purposes, unessential. 



Descriptive. "The origin of the ovary is similar to that of 

 the spermary. Here also, the starting point of the new organ is 

 the interstitial tissue. In a zone which includes nearly half of the 

 circumference of the animal, the cells, which at first lie singly 

 among the neuromuscular cells, multiply to form numerous irregular 

 masses. At the same time they increase in size and the boundary 

 between nucleus and cell body becomes indistinct; at first glance 

 one would think that free nuclei were crowding together. The cell 

 groups grow rapidly, come into contact and fill the room between 

 ectoderm and endoderm, while the neuromuscular cells are crowded 

 out and sideway. So there lies in this space a plate of cells with 

 parallel sides but convex ends. The cells which form the center of 

 the mass cease division and assume a round or slightly flattened 

 form: their plasma clears up and a host of strongly refractive bodies 

 appear which are aggregated about the nucleus. Simultaneously the 

 cells line up in rows converging to a central point, giving to the 

 ovary a radial appearance. At this stage the ovary is an eminence, 

 slightly depressed at the summit, measuring 1 mm long and 025 mm 

 broad: it is readily recognized by its transparent whiteness, con- 

 trasting sharply with the surrounding opaque tissue. The above 



