942 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



theory accounts for the regeneration of lost parts by the assumption that the 

 cells in the vicinity of the wound, by the proliferation of which the new part 

 grows, contain, besides the active determinants that have given them their 

 specific character, other determinants that are latent until the opportunity for 

 regeneration arrives. Some cells do not possess such latent determinants, and 

 hence some parts of a body are incapable of reproducing lost parts. 



Such are the main features of Weismann's theory a germ-plasm of highly 

 complex architecture and independent of somatoplasm ; continuity of germ- 

 plasm and non-inheritance of acquired somatic characters tending to preserve 

 the uniformity of the species ; slight nutritional variation of germ-plasm and 

 sexual reproduction tending to destroy that uniformity ; the result is inherited 

 resemblance and congenital variation. The theory is now being most actively 

 discussed. 



Theory of Epigenesis. Among epigenesists no one theory may be said to 

 be pre-eminent. The main features of the epigenetic conception, already 

 referred to, may be summarized as follows : The fertilized ovum is isotropous, 

 i. e. all parts are essentially alike; germ-plasm probably consists of minute 

 particles, but these particles do not represent definite cells or groups of cells 

 of the adult ; segmentation is a quantitative process ; the early blastomeres 

 are essentially alike, and any one of them, if isolated from the rest, may 

 give rise to a whole organism, although under ordinary circumstances they 

 react upon each other in bringing about the resultant individual ; there is 

 no predetermination, either in the germ-cells or in the segmenting ovum, 

 of the ultimate form or function of the various constituent parts ; morpho- 

 logical differentiation and physiological specialization are phenomena of 

 comparatively late embryonic life, and the prospective character of any one 

 cell, whether it is to be a muscle-cell, gland-cell, nerve-cell, or germ-cell, is 

 determined by the influence of the surrounding cells and the surrounding 

 physical and chemical conditions " the prospective character of each cell is a 

 function of its location." Extreme epigenetic views are not so numerous as 

 those of preformation. 1 



The more moderate thinkers of the present time recognize truth in both 

 preformation and epigenesis, and are endeavoring by experimental methods to 

 determine how much share in the production of the characteristics of the off- 

 spring is to be ascribed to the original qualities of the germ-plasm and how 

 much to the physical, chemical, and physiological phenomena of the immediate 

 environment of the developing embryo. Such experimental work is per- 

 formed at present upon the simpler and lower animals, mostly marine inverte- 

 brates, and has reference to the effect of changes in the composition of the water 

 surrounding the embryo, the effects of various salts, of changes in temperature, 

 of pressure, of electricity, etc., etc. Such work is now in its infancy, but it is 

 doubtless destined to yield results of the highest value in an understanding of 

 the true nature of heredity. 



1 The best statement of a moderate epigenetic theory is to be found in Zeit- und Streitfragen 

 der Bialogie: 1. Fraformation oder Epigenesis t O. Hertwig, 1894. 



