Dr. A. Korotneffow the Embryology of Hydra. 429 



Kerschner • The earliest investigations of Von Siebold and 

 Max bchultze have no particular interest as regards the his- 

 togeny, for which reason I shall confine myself to the above- 

 mentioned memoirs of Kleinenberg and Kersclmer, indicate 

 the contradictions which occur in them, and then turn to my 

 own observations in this department. 



According to Kleinenberg the segmentation is perfectly 

 equal, into 2, 4, 8, 16 parts, and so on, but no segmentation- 

 cavity is formed. Neither the ovum nor the segments origi- 

 |l nating from it possess nuclei ; the latter occur only in the 



tuture cells. The peripheral cell-layer becomes converted in 

 course of time into a chitinous egg-capsule ; and then a second 

 membrane soon appears upon the egg within the capsule. Now, 

 however, commences a period of relative rest, which is to be 

 regarded as a true histolysis : the cells melt away ; the nuclei 

 disappear; and the germ becomes converted into a plasma-body 

 which contains only yelk-nuclei (pseudocells, KL). In the 

 interior of the protoplasm there is then produced a clear ex- 

 centric space, which soon enlarges, and represents the first 

 indication of the stomachal cavity. The external capsule soon 

 splits, and the germ remains bounded only by the inner 

 membrane. It is only now that is produced the first trace of 

 the division of the wall into two layers, the ento- and ecto- 

 derm. The embryo then extends itself, and at one pole its 

 wall becomes very thin ; here a fissure makes its appearance ; 

 and the mouth is thus produced. Now, according to Kleinen- 

 berg, the clear outer layer divides up into separate cells ; at 

 the same time the tentacles make their appearance as hollow 

 processes ; and the embryo soon becomes free. 



The description of the development of Hydra viridts by 

 Kerschner differs essentially from that just given. After 

 segmentation, according to Kerschner, a blastula is formed, 

 whilst according to him there is no morula. Into the seg- 

 mentation-cavity there is, from the pole directed towards the 

 parent animal, an immigration of cells which form the ento- 

 derm. The ectoderm does not disappear during the formation 

 of the chitinous envelope, but is retained. The entoderm, 

 formed as above by immigration, acquires an appearance like 

 that of connective substance, but changing from time to time 

 by the development of protoplasmic cords and the spaces 

 existing between these. 



My own investigations relate chiefly to the development of 



a aurantwea, although I have also observed several 



stages of that of Hydra fusca. As signs of the maturity of 



* Kersclmer, Zool. Anzeig.no. 64 (Sept. 6, 1880), p. 451. 



