12 PROFESSOR ALLMAN. 



division and the nature of protoplasm movements, in which 

 the author contends that the essential distinction between 

 proper contractility as presented by muscle, and the move- 

 ment which shows itself in every other form of protoplasm, 

 consists in the fact that muscular contractility is essentially a 

 motion in definite directions, consisting solely in a shortening 

 of the muscle simultaneously with an increase of its trans- 

 verse section ; while in every other form of protoplasm — in 

 an amoeba, a connective-tissue particle, &c. — every molecule 

 of its mass is moveable in every direction; and since we know 

 that the all-sided mobility is a property of the indifferent pro- 

 toplasm, we must regard muscle-substance as a modification 

 of this, in which there has occurred a peculiar arrangement 

 of the molecules, which excludes all movements but one. 



The views of botanists and animal physiologists regard- 

 ing the nature of cell-division are discussed at length ; 

 and in opposition especially to the hypothesis of Hof- 

 meister, who maintains that cell-division is identical with 

 the formation of drops in a liquid, Kleinenberg regards 

 it as having its origin essentially in a destruction of the 

 uniformity of the protoplasm, local differences in the 

 cohesion of the mass setting themselves up ; in some ])laces 

 the molecules attract one another more strongly, while in 

 others the uniform weak attraction is maintained ; and this 

 cohesion-difference will induce a general or local change of 

 position of the molecules relatively to one another. If we 

 designate by the name of currents the continuous movements 

 of a mass of protoplasm, which are the result of cohesion- 

 differences, we may distinguish in the Hydra egg two forms 

 of currents : 1, those which show themselves in the forma- 

 tion of local and superficial pseudopodia ; and 2, currents 

 which change the whole form of the egg. He thinks it very 

 probable that the same forces which cause the currents cause 

 also simultaneously the division ; and he thus regards the 

 division of the cell as a protoplasm-current phenomenon. 



A very important process in the development of the Hydra 

 germ next begins to show itself. This is the formation of 

 the external covering or shell of the germ. In Hydra viridis 

 this is first seen as an exceedingly thin structureless pellicle, 

 which invests the free ends of the prismatic cells, which, as 

 we have already seen, form the surface of the germ. This 

 pellicle sends in short processes between the cells, and 

 extends uninterruptedly over the whole surface of the germ, 

 from which, by maceration in acetic or very dilute chromic 

 acid, it may be separated as a continuous membrane. The 

 formation of this first pellicle is followed by that of a second 



