434 Dr. A. Korotncff on the Embryology of Hydra. 



After the alteration of the peripheral cells of the hypoblast 

 the same phenomenon is to be observed in the central cells. 

 These begin to divide, and to migrate, as is so frequently the 

 case in the eggs of insects, towards the periphery of the egg*. 

 The division of the cells still goes on, and forms a layer of 

 small cells of the interstitial tissue at the bottom of the ecto- 

 derm. Then the egg-capsule bursts, and the germ, still 

 surrounded by the vitelline membrane, becomes free. A 

 transverse section of this stage shows us the" presence of the 

 membrana propria and the development of the stomachal 

 cavity. The cells of the entoderm are already completely 

 developed and delimited. The stomachal cavity remains 

 still for some time filled with a pasty mass, which contains 

 vitelline globules. 



In Hydra fusca the development of the egg-capsule takes 

 place somewhat differently. We have already seen that her 

 the mucous layer is formed by the parent animal itself. A 

 perfectly smooth egg-capsule soon makes its appearance ; and 

 at the same time the epiblastic nuclei shrivel up. In a short 

 time we see that the hypoblastic as well as the epiblastic cell; 

 form a common mass ; the nuclei of the cells become pale, and 

 afterwards disappear, without inducing a degeneration of the 

 cell-body itself. At this time a vitelline membrane is secreted . 

 After the setting-free of the primary hypoblastic cells a divi- 

 sion takes place in these ; and the newly formed cells migrate 

 to the periphery and constitute the definitive epiblast. As 

 regards Hydra fusca, therefore, it is established that the cell- 

 body of the epiblastic elements is not entirely exhausted with 

 the secretion of the egg-capsule, but, after a conversion, also 

 takes part in the plastic development of the germ. 



After the appearance of an internal cavity the embryo ex- 

 tends itself, becomes perceptibly elongated, and causes the 

 vitelline membrane to burst. Soon afterwards the embryo 

 acquires at one pole a mouth, which originates as a fissure at 

 a thin-walled spot. Immediately after the formation of the 

 mouth the tentacles make their appearance as hollow processes, 

 as described by Mereschkowsky : at first there are two ; and 

 then the others are formed successively in pairs. 



The great importance that Kleinenberg attaches to the dis- 

 appearance of the outer epiblastic layer of the embryo is well 

 known. "When the epiblast is cast off, the nervous layer makes 

 its appearance ; and hence the ordinary ectoderm of Hydra 



• The setting-free of the cells from the vitelline halls was observed by 

 Tichomiroff in the development of Bombyx mori\ I have also seen it in 

 Bryozoa in the development of the statoblast. 







