224 



THE AMERICAN MONTHLY 



[December, 



by Prof. E. Ray Lankester and 

 others, but is still strenuously upheld 

 by some observers. Dr. Hamann. 

 in recently published observations, 

 agrees with Brandt that whenever 

 chlorophyll is present in animals we 

 have to do with independent unicel- 

 lular algcE. He states positively that 

 he has removed the green bodies from 

 H. viridis and has cultivated them 

 in water, where they grew vigorously 

 and multiplied rapidly by repeated 

 division. In the same journal in which 

 these results are given (Zeitschr. wiss. 

 ZocJ.,xxxvii,p. 457) 1 a directly oppo- 

 site view is advocated by Marshall, 

 who kept a green Hydra for 6 weeks 

 in the dark withovit any change in 

 these granules being perceptible. 

 He therefore concludes that they 

 are not alg^e but characteristic of 

 the animal itself. The question is 

 evidently still attracting considerable 

 attention. 



The outer membrane of Hydra., 

 the ectoderm (fig. 19 e. c.) , is a some- 

 what more complex structure. Klein- 

 enberg finds that externally it is com- 

 posed of a layer of large cells of 

 nucleated protoplasm. Below these 

 and running up between them are 

 numbers of small cells with compar- 

 atively very large nuclei, constituting 

 what he names the interstitial tissue, 

 (fig. 20 i. t.), in which the so-called 

 nematocvsts (or thread cells) and also 

 the reproductive bodies are formed. 



Finally, beneath these and in con- 

 tact with the endodermal layer is a 

 narrov\^, clear zone, in w'hich con- 

 tractile fibrillEe are imbedded (figs. 

 19 and 20 m. l.) Kleinenberg makes 

 the very important statement that 

 these fibrillcB are processes of or ex- 

 tensions from the above-mentioned 

 laver of large external cells. That 

 is to say, the ectodermal cells taper 

 towards the endoderm, being pro- 

 longed into fine processes which fre- 

 quently divide, and when these pro- 

 cesses meet the endodermal la3-er they 

 bend sharply at right angles and run 

 parallel with the long axis of the body 

 (fig. 21). These fibres are bound to- 



gether by a soft, colorless connective 

 tissue into a thin membrane which 

 is every^vhere interposed between the 

 endodermal and ectodermal layers. 



The careful and ingenious experi- 

 ments of Kleinenberg and others 

 leave scaixely a doubt that it is to 

 this thin layer of so-called ' muscle 

 processes ' that the great contracti- 

 bility of Hydra is solely due. And 

 while the muscle processes are highly 

 contractile, the large ectodermal cells 

 of which these processes are mere 

 continuations remain perfectly pas- 

 sive. Therefore the entire cell can 

 certainly not be called a muscle cell. 

 How, then, is it to be regarded.? 

 Kleinenberg thinks the only logical 

 conclusion to be arrived at is to con- 

 sider these ectodermal cells of Hy- 

 dra, for which he proposes the name 

 of ' neuro-muscle cells,' as the lowest 

 developmental stage of the nervous 

 and muscular systems which has yet 

 been discovered, the component parts 

 of nerve and muscle not having yet 

 been differentiated as in the case of 

 the higher animals, but each single 

 cell exercising a double function, in- 

 asmuch as the long processes contract 

 and act as muscle, whereas the cell- 

 bodies from which they emanate act 

 as motor-nerves by receiving a stim^ 

 ulus from the surrounding medium 

 and transmitting this to the fine pro- 

 cesses, causing these to contract. He 

 points out that nerve and muscle are 

 always coexistent ; that they are 

 tunctionally dependent upon each 

 other ; that Hydra shows absolutely 

 no trace of a separate nervous system, 

 but it does possess a muscular tissue 

 in the shape of contractile fibres, 

 which fibres are, however, only the 

 prolongations of the large ectodermal 

 cells. The latter are demonstrably 

 7zot contractile. They form the en- 

 tire external boundary of the animal. 

 All outside impulses, therefore, can 

 only reach the muscular layer by 

 striking the external non-contractile 

 cells and being transmitted by them. 

 He therefore believes the neuro-mus- 

 cle cell of Hydra to be the starting 



