124 DEVELOPMENT OF THE FROG'S EC G [Ch. XII 



its environment is self-evident; a certain amount of warmth 

 and of oxygen, etc., must be present. These, while necessary 

 for the development of the egg, do not necessarily determine 

 the sequence of events ; for under the same external conditions, 

 eggs of different animals develop very differently. The results 

 obtained by placing the frog's egg under different conditions 

 also show that the power of progressive development must lie 

 within the egg itself. Roux compared the egg, in this respect, 

 to a complicated piece of machinery which, when once set in 

 motion, would go through a long series of changes depending 

 on its internal structure. 



If so much be granted, the next question to be answered is 

 this : do all the parts of the dividing egg work together, i.e. in- 

 teract to form the result, or have the parts of the egg separated 

 from one another by the cleavage the power to develop inde- 

 pendently? The first alternative Roux called the differentiat- 

 ing interaction of the parts, and the latter alternative, the self- 

 differentiation of the parts. With reference to the results of 

 the experiment in which one of the first two blastomeres of the 

 frog's egg was killed or injured, Roux concluded that each of 

 the first two blastomeres shows in this experiment the power 

 of self-development : i.e. each half is independent of the other 

 and we may legitimately infer that when both blastomeres are 

 alive, as in the normal development, the same self-differentia- 

 tion of each blastomere takes place. This independent devel- 

 opment goes on till the organs of the body begin to form. 

 Whether the limit of independent development is then reached 

 we do not know, for it is possible that in the complicated series 

 of movements that take place in the formation of some of the 

 organs, the power of independent development may be sup- 

 plemented or replaced by the action resulting from the cor- 

 relation of the parts to one another, i.e. by a mechanical 

 interaction of different parts. Each of the first two blasto- 

 meres contains not only the building-material for the corre- 

 sponding parts of the embryo, but also the differentiating and 

 formative forces for those parts. The cleavage in the direct, 

 or normal development of the individual, divides qualitatively 

 the " germ-plasm," and, in particular, the nuclear material. 

 The development of the frog's gastrula and of the embryo 



