ON THE RELATION BETWEEN, ETC. 373 



logical problems it is advisable first of all to study the 

 primitive conditions exhibited by the lower cells, such, for 

 instance, as are revealed to us by masses of naked pro- 

 toplasm, e.^o-,^ by the Amoeba. Such cells, in spite of, or 

 rather just because of, their varying form appear to me to be 

 capable of explaining not a few of the elementary problems 

 of the organic formation of the body. 



The organic form of the Amoeba is controlled by two 

 principal factors, one physical and the other chemical. 

 Physically, the Amoeba cell possesses the qualities of a fluid. 

 This drop of fluid, however, is constantly exchanging its 

 matter with the surrounding medium. As a fluid, the body 

 of the Amoeba is subject to the physical laws which govern 

 fluids and, if considered as existing solely under the in- 

 fluence of these laws, could well be supposed to assume a 

 definite fixed form, let us say that of a sphere. This form the 

 body of the Amoeba would permanently retain if the condi- 

 tions remained unaltered. Change, however, will take place as 

 soon as it enters into chemical relations with the surroundinof 

 medium. In these chemical relations, in other words in meta- 

 bolism, then, we have the cause of the movement, of the 

 flowing onwards and of the changes of shape of the Amoeba. 



It is worth while to look a little more closely into the 

 relations between the metabolism of the Amoeba and its 

 change of shape. Let us take first, by way of example, the 

 relation between respiration and the change of form. In 

 connection with a previous observation by Ktihne, I have 

 elsewhere shown that a constant connection exists between 

 the extension of the pseudopodia of the Amoeba and the 

 amount of oxygen in the medium. With the withdrawal of 

 oxygen the formation of pseudopodia and consequently the 

 change of shape ceases ; on the return of oxygen it re- 

 commences. This fact appeared to me to be of such funda- 

 mental importance that I repeated Kuhne's experiments on 

 large Rhizopods from the Red Sea, these being in many 

 respects specially suited for the purpose. My results agreed 

 entirely with those obtained by Ktihne. The expansion of 

 the long, straight, thread-like pseudopodia, such as the long 

 rays which stream outwards from the cell of the Orbitolites, 



