294 NATURAL SCIENCE. Oct.. 



material, because in cell-division there is always an equal division. 

 No doubt sexual cells, for the most part, arise only in special parts of 

 the organism. But, in these cases, the fact that certain regions of the 

 body give rise to the sexual organs is merely a part of the general 

 ordering of the body into tissues and organs, and is not due to the 

 presence in sexual cells of any special plasm. Hence it is not surprising 

 to find in special cases diffused powers of reproduction or of regenera- 

 tion, any more than it is surprising to find muscle-cells or gland-cells 

 in unwonted situations. 



In one matter, the possibility of the inheritance of acquired cha- 

 racters, a question which bulks more largely in popular controversy than 

 in Weismann's theory, the two views do not differ theoretically. 

 Weismann admits the possibility of the influence of outside forces 

 modifying the germ-plasm in the cells, and as Hertwig's specific 

 material is diffused through all the cells of the body, it too is not 

 removed from the possibility of being altered by the environment. 

 Hertwig, however, does not specially deal with the controversy as it 

 is known, but one draws the inference from the general tendency of 

 his views, that his specific material is supposed to have as much 

 organic stability as the germ-plasm. 



The epigenetic part of Hertwig's views about development con- 

 sists of an account of many of the factors, apart from a specific plasm, 

 which direct the development. The leading motive in his account of 

 these is one diametrically opposed to Weismann's view that a cell 

 must become what it is, and develops only in accordance with the 

 determinants which it received from among the determinants in the 

 parent cell from which it arose. Hertwig believes that a cell becomes 

 whatever it is made to become by the forces round about it. The 

 limit to this is given by the specific material with which it is endowed. 

 A cell of one animal could not be made into the cell of another, but 

 the cell of an animal might have been any other cell of the animal 

 had its environment been different. In the words of Driesch, the 

 specialisation of a cell is a " function of the locality of the cell in the 

 organism." The causes of the elaboration of an organism from 

 its germs or incipia lie in conditions outside the incipia in the 

 egg-cell, but which, nevertheless, follow in regular and orderly 

 sequence. The first of these causes is the continual change in 

 the relations of cells to each other and to the external world as 

 the organism is built up by cell-multiplication. Physiologically 

 speaking, the differentiation of cells in different directions is due to 

 the reaction of the organic substance to different external stimuli. 

 The capacity of the cell to assimilate or to go through metabolic 

 changes in the case of the fertilised egg-cell produces the cleavages 

 by which the morula is formed. The particles of the protoplasm 

 were arranged round the nucleus of the egg-cell as a single centre of 

 forces. In the morula they are arranged round a number of new 

 centres corresponding to the new nuclei. Thus, even in equal-heirs 



