430 Dr. A. Korotneff on the Embryology of Hydra,. 



the ovum we must regard two direction-vesicles, in which 

 there are frequently one or two vitelline globules. These 

 vesicles do not appear as derivatives of the division of one 

 vesicle into two, but as independent formations at different 

 points of the surface. At first the egg stretches itself out, but 

 soon resumes its previous spherical form. Soon afterwards 

 the germinal vesicle divides — a phenomenon which Kleinen- 

 berg has entirely overlooked, and I have myself only super- 

 ficially observed it on account of the opacity of the egg. At 

 the pole of the egg turned away from the body of the Hydra 

 there appears a shallow depression, the margins of which form 

 numerous pseudopodia. The further this furrow penetrates 

 downwards the narrower it becomes, so as to form a fissure, 

 which appears at the bottom as a narrow transverse canal. 

 Finally the egg divides into two spheres, which are quite 

 separate and only touch each other. Soon, however, the two 

 spheres lay themselves closer together in the direction of their 

 length, and then a division of the two nuclei takes place ; after 

 this a transverse fissure makes its appearance, which causes a 

 separation of the egg into four spheres. The third divisional 

 plane is equatorial ; this produces eight spheres, which, how- 

 ever, soon contract in such a manner that the germ again 

 becomes globular* The fourth and fifth divisional planes are 

 so situated that they form two grooves running at equal dis- 

 tances from the equator ; by this means we get a germ which 

 consists of 16 cells. Now the first trace of an internal cavity 

 is to be observed ; it is produced because the inner extremities 

 of the germ-cells are not in close contact, but have a small 

 space free, which Kleinenberg overlooked but Kerschner has 

 mentioned. 



Hitherto the segmentation of the egg has taken place quite 

 regularly ; but now an irregularity is to be observed. This is 

 most strongly marked in Hydra fusca 7 and especially in the 

 cells which remain most intimately connected with the body of 

 the parent *. While the cells which form the dome of the 

 blastula become comparatively small by more rapid division, 

 these large basal cells remain undivided, or, more properly, 

 are retarded in their division; but then they become most 

 active in the development of the embryo; they divide in a trans- 

 verse and longitudinal direction, and form a second hypoblasts 



eneration of cells, which occupy the primitive Baerian cavity. 



n this second cell-generation also large elements are to be 

 observed, which must be regarded as a derivative of the basal 



Further on it will be shown how long thia connexion with the parent 

 organism persists. 



