64 ELLIOT R. DOWNING. 



11. The egg thus increases greatly in size and with the mass 

 of interstitials about it, greatly distends the ectoderm cells which 

 elongate into fibers and are crowded to one side. 



12. The egg breaks through these restraining ectoderm cells 

 which are transformed into lamellae at their peripheral end but 

 are still connected with the muscle layer of the mesoglcea. 



13. Brauer ^ describes the newly extruded egg as spherical, 

 except for a stalk which attaches the egg to the parent. There 

 is, too, a conical depression in the peripheral layer of the egg 

 which layer is free from yolk. This depression lies opposite the 

 attaching stalk over the nucleus, and by it the sperm has access. 



14. Brauer states that the hydra egg gives off two polar bodies. 

 In attempting to confirm these results on sections of hydra 



prepared while working on another hydra problem, evidence 

 accumulated that seemed to contradict some of these results. 

 Additional material now convinces me that some of these state- 

 ments are incorrect for //./?/iTrt and permits me also to settle 

 some additional points, notably on the chemical nature and the 

 exact method of the inclusion of the yolk. 



1. The multiplication of interstitial cells at the site of the ovary 

 is by mitosis, twelve chromosomes appearing in the figure. 



2. The interstitials are no less frequent immediately about the 

 ovary than elsewhere, so there is no evidence of their migration 

 into the region of the ovary. 



3. The cells at the center of the ovary are larger than those at 

 the margin, as Kleinenberg states. My measurements show that 

 the central cells average four times, or a trifle more, the volume 

 of the marginal. It is to be remembered that these central cells 

 are adjacent to the growing eggs. The influences causing growth 

 seem to operate on the whole region. 



4. Kleinenberg, working largely on //. viridis, and later 

 authors, notably R. Hertwig, working on H.fusca, claim that the 

 egg appears only after the ovary has achieved considerable size. 

 My sections seem to force the conclusion, however, that the egg 

 is always present, before proliferation of the interstitials begins to 

 form the ovary. The egg is often, and so far as I can see always, 

 growing rapidly before the interstitials begin to multiply. It 



^Zeit.f. luiss. Zoo!., V., 52, pp. 167-216. 



