Dr. A. Korotneff on the Embryology o/*Hjdra. 433 



putably described the interspaces of the processes as vacuoles, 

 and the spines or processes as the walls of the cells. Accord- 

 ing to his description the entire outer cell-layer of the germ 

 becomes converted into a hard rigid structure, the egg-shell 

 of the embryo ; in Hydra viridis a complete conversion of the 

 cells into a capsule occurs. According to Kerschnerthe ecto- 

 derm does not become converted into the chitinous envelope, 

 but is persistent. 



The further description will fully explain our own stand- 

 point upon this subject. After the separation of the external 

 spiny shell the embryo contracts, and soon secretes a second 

 extremely thin membrane ; according to Kleinenberg this 

 membrane is produced by the hardening of a tluid which is 

 secreted by the germ between its surface and the inner wall 

 of the egg-shell*. 



The epiblastic cells, after the secretion of the egg-capsule 

 and vitelline membrane, are subjected to a retrogressive altera- 

 tion : they soon acquire a coarsely granular texture ; then- 

 nuclei become strongly refractive, drawn out in length, and 

 curved. In proportion as the vitelline membrane thickens 

 and becomes refractive the primitive epiblastic cells become 

 smaller and smaller ; they therefore undergo a degeneration 

 until scarcely any trace of them is to be seen. 



The nuclei of the hypoblastic cells at the same time divide ; 

 and this process is most strongly manifested at the periphery 

 of the egg, at the bottom of the epiblastic cells. Soon these 

 cells occupy the place of the primitive epiblastic cells, and 

 form the secondary epiblast, or the definitive ectoderm of the 

 Hydra. The succeeding stage has no longer the smallest 

 trace of the primitive epiblastic cells. The demonstrated facts 

 prove the following proposition : — The 'primary epiblast is 

 completely used up in the formation of the egg-capsule, the 

 vitelline membrane, and the mucous layer, and takes no part in 

 the development of the secondary epiblast 



* VonSiebold ( 4 Lehrbuch der vergleiehenden Anatomie/ 1848, p. 51) 

 describes the egg of Hydra vulgaris as enveloped by a delicate cobweb 

 membrane, which issues from tlie cup-shaped organ. Before sue] an 

 egg separates from its parent stock the envelope which immediately sur- 

 rounds the vitellus acquires a firm consistence; and is at the same time 

 coated by a gelatinous mass; then, in Hydra vulgaris, obtuse processes 

 grow out from it all around, which lengthen, divide once or more at their 

 apex, and thus acquire a pronged form. This description, according to 

 Kleinenbenr, is quite incorrect ; but it seems to me possible to brin -iese 

 facts in accordance with my own observations. The cobweb membr.uie 

 is probablv the extended cellular envelope belonging to the maternal 

 organism which covers the mature egg before fertili lion ; the gelatinous 

 mass is secreted in Hydra fusca from the glandular pedestal, and covers 

 the egg, as has already been stated, even in the latest stages. 



