84 PHYSIOLOGY CHAP. 



protoplasm shall have attained a certain degree of organisation, 

 whether chemico-physical or morphological, so as not to be homo- 

 geneous in any individual particle. We saw in Chapter I. 

 that the minimal degree of organisation necessary to constitute 

 a complete organism is in all probability represented by its 

 differentiation into cytoplasm and nucleus. A homogeneous bit 

 of living matter detached from an elementary organism, either of 

 cytoplasm or of nucleus, is incapable of prolonged existence ; on 

 the other hand, a minute particle of heterogeneous protoplasm, i.e. 

 a fragment of nucleus and cytoplasm combined (p. 13), is capable 

 of nutrition, integration, and reproduction. Vital metabolism 

 cannot continue without the natural union of these two essential 

 parts of the cell, showing that there exist between them reciprocal 

 exchanges of matter and energy the universal internal condition 

 of cell life. 



The same applies to complex or multicellular organisms. In 

 the lowest grades of organisation, it is possible to divide a multi- 

 cellular individual into one or more segments without fear of 

 killing it : each segment continues to live, to show signs of 

 sensibility, to grow and regenerate into a new individual like that 

 of which it was originally a part. The classical example of this 

 marvellous phenomenon was given for the first time on animals, 

 in 1774, by the Genevese naturalist Trembley, with fresh- water 

 polyps. The interpretation now accepted is that the cells of which 

 the polyp is built up are not too highly differentiated in structure 

 and function to be capable of mutual substitution ; each cell, i.e., 

 represents the germ of the entire polyp, and can therefore recon- 

 struct it. 



In the higher grades of organisation, on the contrary, where 

 there is a more or less advanced differentiation, both morphological 

 and functional, of the parts that constitute the organism as a 

 whole, it is no longer possible to multiply the individual by 

 sections, because the life of each part is conditioned by that of the 

 others, and they all represent integrating factors of more or less 

 importance to the life of the aggregate. In the higher vertebrates 

 also it is possible to amputate a limb or an organ, or even several 

 limbs or organs, without necessarily causing death. This is either 

 because the function of the lost parts can be replaced by others, or 

 because there is not between the missing parts and those which 

 remain a reciprocal exchange of matter and energy sufficient to 

 make them indispensable to the life of the entire aggregate, in the 

 way that the nucleus is necessary to the life of the cytoplasm 

 and the cytoplasm to the life of the nucleus. 



We know nothing positive in regard to the conditions, the 

 internal stumuli, and the intimate mechanism of the phenomena 

 of the nutrition and growth of protoplasm, and of cellular repro- 

 duction or neo- formation. All our ideas on this subject are 



