138 The Science of Life. 



any more than notochord and backbone are; and he 

 extended this to the nervous system of vertebrates a 

 difficult path which Dr. Beard has followed. 



The newest departure in embryological investigation 

 has been along experimental lines, and there is no better 

 Experimental illustration of modern biological activity. 

 Embryology, within a few years a vast literature has ac- 

 cumulated, an important journal Roux's Archiv fur 

 Entwicklungsmechanik has arisen as a specialized 

 record of research, and there is already a text-book 

 (Haacke's) on the subject. The investigations are still 

 too novel and incomplete to be securely appreciated, but 

 there can be no doubt that they have shed fresh light 

 on old problems, and that they are full of promise. It 

 seems fair to associate one name in particular with this 

 new movement that of Wilhelm Roux, the keen-witted 

 author of Der Kampf der Theile im Organismus (1881) 

 The struggle of parts within the organism, but his work 

 has been ably criticised, or supplemented, or extended, 

 as the case may be, by Oscar Hertwig, Born, Chabry, 

 Driesch, Herbst, Morgan, Wilson, and others. The 

 experimental work is especially of two kinds: (i) sub- 

 jecting developing ova to new conditions of chemical 

 medium, pressure, gravity, temperature, &c. ; (2) punc- 

 turing or isolating certain cells of the segmenting ovum 

 and observing results. The results have immediate 

 relation to several problems : (a) the morphological 

 problem of cell-lineage, (b) the physiological problem of 

 immediate growth-conditions or body-physics, (c) the 

 theory of development, and (d) the influence of the 

 environment in inducing modifications. 



There are at present two main theories of development 

 the mosaic theory of Roux and Weismann, and the 

 Theories of anti-mosaic theory of Hertwig and Driesch. 

 Development. i n their extreme forms these two theories 

 are irreconcilable, but with mutual concessions it seems 

 possible to combine them. 



According to the mosaic theory, the cause of differen- 

 tiation is to be found in the nature of cell-division, which 

 is supposed to be qualitative, sifting out different char- 

 acteristics into the two daughter-cells. Thus if the 



