642 



Chas. B. Wilson 



the amphibian embryo responds to variations in the surrounding 

 medium. 



These experiments also bear significant testimony in another 

 direction. The early differentiation of cells as to function, and their 

 consequent interdependence has been admirably worked out by 

 eminent investigators in the recent morphological examination of 

 different embryos (37 and 48), but it scarcely admits of physiological 

 proof under normal conditions. 



Under these abnormal conditions, however, a comparison of 

 experiments indicates quite clearly that in the unsegmented egg there 

 is very little differentiation. If there be any at all it is a very 

 plastic one, and accommodates itself easily to changed conditions. 

 But as soon as segmentation begins differentiation commences also, 

 and the egg acquires thereby definite regional distinctions. 



These increase with advancing development and at the 64-cell 

 stage they become sufficient to hinder the egg perceptibly in accom- 

 modating itself to a new environment. 



In the blastula stage they furnish a serious obstacle to any 

 such accommodation, and by the time the blastopore is fully formed 

 they have become so well established that they cannot be altered, 

 and consequently the organism must die under the influence of any 

 serious change in its environment. 



Hertwig has clearly shown that the cell material which is 

 used in these embryos for the origin and development of the dorsal 

 organs and the tail is differentiated in a very different manner from 

 that which occurs in normal development. 



The same truth has been shown in the present experiments 

 when discussing the fate of the blastopore. It is this readjustment 

 of differentiation which proves so difficult for the embryo, since 

 material which has been gradually separated for a particular purpose, 

 or at least for the accomplishment of that purpose in a particular 

 way, must be used for a different purpose or for its accomplishment 

 in an entirely different way. This subject of differentiation during 

 development, and of the potence of the early blastomeres, has been 

 eminently worked ont by Wilson (49) and Morgan (38) in very 

 recent publications. They both frankly state that at present we do 

 not know how such differentiation is brought about. Morgan » thinks 

 there are sufficient reasons to conclude that the power of differen- 

 tiation lies within the egg itself, and does not depend directly on 

 external stimuli« (38, pag. 134). 



