432 Dr. A, Korotneff <m the Embryology of Hydra. 



formed by each of its cells. The outer border of the epiblast- 

 cells with the processes acquires a yellowish shining border, 

 the indication of a chitinous egg-shell. Each epiblast-cell 

 then represents a cylindriform body, at the bottom of which 

 various vitelline globules are to be observed; further outward 

 is the nucleus ; the above-mentioned process is clear and is 

 formed of clear plasma, while the plasma of the cell itself is 

 turbid and granular. At this time the egg separates from the 

 body of the parent and adheres to various objects, a portion of 

 the epiblast-cells undergoing a transformation. Of the cells 

 which, as an uninterrupted layer, surround the embryo and 

 possess processes, a portion acquires a turbid and coarsely 



granular appearance by a glandiform change which they 



undergo. At the same time also the form of these cells be- 

 comes altered ; they become elongated, and so form a disk 

 which gradually increases in size by multiplication of the 

 cells, and at the same time spreads over a part of the surface 

 of the germ. It is this disk by which the embryo attaches itself 

 alter separation from the body of the parent. 



The glandular cells of the disk take part in the secretion of 

 a sticky, mucous substance ; when this is formed the cells be- 

 come diminished in size, and no longer differ from the ordi- 

 nary cells. The secretion of the sticky substance takes place 

 gradually, and so that the substance appears to be composed 

 of layers. It lies beneath the chitinous egg-shell and separate 

 this from the cells. 



In this stage the egg becomes perceptibly smaller, and a 

 metamorphosis or histolysis of the hypoblast commences. The 

 cells of the hypoblast lose their sharp demarcation from one 

 another ; their plasma, however, has become concentrated 

 around the nuclei, and, as it were, ejected the vitelline globules ; 

 so that these are quite separated from the cells, and lie as it 

 were outside of them in the intervals between them. The 

 demarcation of the epiblast and hypoblast no longer appears 

 so distinctly as before. 



According to Kleinenberg the whole business of the forma- 

 tion of the egg-shell in Hydra aurantiaca takes place as 

 follows : — Immediately beneath the free surface of the cells 

 there is produced a space of lenticular form filled with fluid; 

 its outer wall consists of an extremely delicate membrane, 

 which is raised from the substance of the cell and seated upon 

 it like a very convex watch-glass ; its bottom is formed by a 

 shallow impression of the body of the cell. The vacuoles 

 soon lose their convexity; but the septa remain, and in this 

 way form the plasma processes or spin s with which the shell, 

 as described by us, is surrounded. Kleinenberg has indis- 



