ANATOMY AND DEVELOPMENT OF HYDRA. 5 



only one function. It appears to him as the most logical 

 view to regard this tissue of the ectoderm of Hydra as the 

 lowest stage of development of the nerve-muscle system in 

 Avhich an anatomical differentiation of the two elements, as 

 this occurs in all the higher animals, has not yet taken place. 

 Every cell is thus the bearer of a double function, those parts 

 which constitute the long processes and lie upon the inner 

 side of the ectoderm being contractile and performing the 

 function of muscles, while the cell-bodies from which these 

 proceed, and which stand in immediate relation to the 

 surrounding medium, receive the stimulus and by transference 

 of this upon the processes effect the contraction of the latter ; 

 in other words, they operate as motor nerves. He proposes, 

 therefore, for these cells the name of '^ nervo-muscle cells." 



If these vicAvs be accepted, a step will at once be gained 

 towards the solution of one of the most important 

 questions with which modern physiological research has 

 occupied itself, namely, the nature of the motor-nerve termi- 

 nations throughout the animal kingdom. While, however, 

 we freely admit the care with Avhich our author has worked 

 out the structure of the parts, and the consistency of the views 

 which he has founded on such structure, we cannot regard 

 these views as supported by reasons so strong as to induce us 

 unhesitatingly to accept them. Kleinenberg supports his 

 doctrine on the assumption that nerve and muscle are corre- 

 lative and inseparable. The fact, however, of our always 

 finding them so associated in the higher animals affords no 

 proof of such a connection being necessary in the lower, 

 while the direct morphological continuity of nerve and muscle 

 has not been established by any of the repeated histological 

 researches Avhich have been especially brought to bear on the 

 elucidation of this very point. It is almost certain that the 

 fibrillae in the hydroid body constitute a contractile tissue ; 

 but that the superficial cell-stratum, from which Kleinenberg 

 regards them as direct prolongations, represents a true 

 nervous system, is still nothing more than a reasonable 

 hypothesis. 



The sketch now given of Kleinenberg's researches into the 

 anatomy of Hydra will show that, while he has confirmed 

 many of the statements of former observers, he has shown 

 the incorrectness of others, and has, at the same time, ad- 

 vanced our knowledge of the animal by the discovery of 

 several new and important points in its structure. 



He next takes up the subject of development, and here his 

 memoir is even richer in new facts and deductions than that 

 part of it which dealt with the anatomy. 



