Dr. A. Korotneff on the Embryology of Hydra. 431 



cells of the blastula ; these divide rapidly, and thus cause the 

 complete filling-rap of the previously existing cavity. In this 

 way we arrive at a^ stage which is apparently analogous to 

 a morula, but, as Kersclmer correctly remarks, is not a true 

 morula ; for it is a secondary form which appears, not before, 

 but after the formation of the Baerian cavity. 



The upper cells of the germ, forming the dome, have no 

 part to play in the production of the hypoblast, and persist 

 without any alteration. The stage just described is a true 

 transition form from a planula to a gastrula ; the function of 

 producing the hypoblast-cells is here assumed by the basal 

 cells without their forming an invag-ination-sac. We have 



& "" «*»«6 



only to acid that in the blastula the cell-nuclei are composed 

 of strongly refractive granules, from which a number of radia- 

 ting streaks run outwards. The second stage has already 

 compact nuclei, in which staining distinguishes no nucleoli. 

 The nucleus itself is enveloped by a quantity of quite clear 

 and transparent plasma * but the chief mass of each segment 

 consists of vitelline globules * of very different sizes. 



After the internal cavity of the embryo is completely filled 

 up, the previous egg-segments acquire the appearance of true 

 cells ; and these divide most rapidly in the outer epiblastic 

 layer. We have now to mention a difference between Hydra 

 aurantiaca and Hydra fusca. In the latter species we see the 

 egg completely adherent to the parent-body, and it is not the 

 egg but the free-swimming embryo that escapes from the 

 parent Hydra after the bursting of the egg-capsule. In Hydra 

 aurantiaca things take place as follows : — The ectodermal 

 cells of the mother which are in contact with the egg gradually 



acquire a gland-like nature and take part in the formation of 

 a pedestal or cup-like organ, which furnishes a sort of mucous 

 substance, serving to effect the adhesion of the egg and to 

 form a special layer around it. In Hydra aurantiaca a struc- 

 ture further occurs which effects the adhesion of the egg, not 

 to the body of the parent, but to various plants or to the glas3 

 plates of the aquarium. This structure in Hydra aurantiaca 

 proceeds from the embryo itself. The egg shows a separation 

 of theepiblast and hypoblast. The greatest alteration is to 

 be observed in the epiblast, the surface of which is no longer 

 smooth, as before, but has acquired a tubercular appearance; 

 at the same time a pseudopodiunvlike lobiform process is 



* In mv previous memoir (Nachr. fur Liebh. der Natur, Bd. xxvii. 

 Moscow, 1860) I have shown that these vitelline globules are to be 

 regarded as metamorpho- d nuclei of the eell-masa which serves for the 

 formation of the esrir. 



