ANATOMY AND DEVELOPMENT OF HYDRA. 3 



may iiqav regard as established in the endodenn of Hydra. 

 Most of them, it is true, have ah'eady been made known by 

 other observers, but it is, nevertheless, a matter of no little 

 value that they should be thus confirmed by the very careful 

 researches of Kleinenberg. 



While the endoderm thus forms a simple cell layer, the 

 ectoderm on the other hand is a complex membrane. It is 

 in Kleinenberg's statements regarding the structure of this 

 part of the Hydra body that the most original results of his 

 anatomical investigations are to be found. The methods 

 which he has employed with most success in his examination 

 of the ectoderm consist in making transverse sections of the 

 animal after it has been hardened by being allowed to remain 

 during a period varying from one to three days in a solution 

 of chromic acid of 0'025 per cent., and then steeping these 

 sections for a quarter or half an hour in dilute acetic acid 

 0*25 — 0"05 per cent. The preparations may then be coloured 

 with fuchsin and preserved in dilute glycerine. 



In sections thus made we find externally a simple layer 

 of large cells, which are in the condition of solid protoplasm 

 masses, with large ellipsoidal nuclei. These cells touch one 

 another only by their broad bases, which are turned towards 

 the surface of the body ; while lying below and running up 

 between them there is a multitude of smaller cells, some of 

 which contain a thread-cell, some only a well-marked 

 nucleus; finally, lying beneath these, and in close contact 

 with the endoderm, is a narrow clear zone in which the 

 well-known fine muscular fibrillse are imbedded. 



The very important statement is now made that each of 

 the large cells which form the most superficial part of the 

 ectoderm, tapers away tOAvards the centre of the section and 

 either terminates in a single process or becomes cleft dich- 

 otomously, so that the inner end of the cell appears many- 

 times branched. The cell-processes thus formed all run 

 towards the endoderm ; when they meet this the finest of 

 them bend sharply at a right angle, and thus form the simple 

 layer of longitudinal soft fibrillee which lies in contact with 

 the endoderm. 



To these fibrillae he assigns the name of muscle-processes 

 (Muskelfortsatze). They are all bound together by an 

 abundant intervening substance into a continuous lamina, 

 which is everywhere included between the endoderm and 

 the ectoderm. The substance by which the fibrillar are bound 

 together not only fills the spaces between the fibrillae, but, 

 increasing in volume tOAvards the endoderm, forms a con- 

 tinuous thin membrane, which by maceration and tearing 



