he pr on UCTION. 505 



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 O. Hertwig : The 

 Biological Problem of To-day ; Preformation or Epigenesis f Authorized translation. 



