534 GENERAL PHYSIOLOGY 



(74) regarding '' organ-forming germ-regions." Roux ('95) and 

 Weismann ('92, 2) are the latest defenders of this view, which affirms 

 that different areas are present in the egg, which in continued 

 division are transferred to different cells, and each one of which 

 affords the material for the development of very definite tissues 

 and organs. In other words, the rudiments of the different parts 

 of the body of the adult organism exist, separate from one another, 

 in different parts of the egg. The chief supports of this view are 

 the results of the experiments that Roux has performed upon the 

 frog's egg, in which he observed that after artificial destruction of 

 one of the first two cleavage-cells, from the other at first only half- 

 embryos developed, i.e., embryos in which one half of the body was 

 wholly wanting, this half being capable of development later by 

 '•'post-generation," as Roux expresses it. In contrast to this is the 

 view of another class of experimenters, especially Pfliiger ('83, '84), 

 0. Hertwig ('92, '93), and Driesch ('92, 93), who deny the existence 

 of organ-forming germ-regions, and believe that the differenti- 

 ation of the homogenous egg-cell into the various kinds of cells is 

 brought about solely by the influence of external factors upon the 

 various substances contained in the egg. Thus, in eggs like that 

 of the frog, which contain substances of different specific 

 gravities — in the frog's egg there is a white substance that is 

 richer in yolk and a pigmented substance that is richer in 

 protoplasm — gravity acts in such a manner as to lead to polar 

 differentiation, so that the heavier substance comes to lie below, 

 the lighter above, and, when the egg is turned, the substances move 

 correspondingly. At the first division of the frog's egg, the polar- 

 differentiated cell becomes divided by a vertical groove into two 

 equal halves, each of which contains white substance and black 

 substance equally. But when Pfluger put frogs' eggs into an 

 abnormal position and fixed them there, the eggs in cleavage 

 frequently segmented into two unequal parts, one of which 

 contained pre-eminently the light, the other the dark mass; never- 

 theless, normal larvae developed. The contents of the egg can, 

 therefore, not be so difierentiated beforehand, that from each part 

 certain organs only can develop ; on the contrary, the different areas 

 in the egg must be wholly similar as regards development. The 

 fact observed by Hertwig speaks a priori in favour of this view, 

 namely, that even single small pieces of the ovum, if they are 

 capable of life and are fertilized, develop into whole individuals. 

 Moreover, in opposition to the observations of Roux, Driesch 

 discovered in the eggs of sea-urchins that from each of the first 

 two, four or eight cleavage-cells, when he had isolated them from 

 one another by shaking, complete individuals always developed, 

 which were distinguished from normal ones by their small size 

 only ; this fact has since been confirmed by numerous observers 

 upon various species of animals, among others by 0. Hertwig upon 



