474 WM. E. KELLICOTT 



meability of the egg membrane is another factor in producing 

 blastoly sis. Thus through the operation of such factors together, 

 some parts of the germinal substance may ''be entirely destroyed 

 owing to the increase in osmotic pressure, while the remainder 

 may go on developing and eventually give rise to various 

 monstrosities" (p. 240). 



The first part of Werber's hypothesis, namely that the effects 

 are due to a physical and chemical modification of the germ 

 cells, cleavage group or germ-disc, is clearly in accordance with 

 the more general disorganization hypothesis. But in the ab- 

 sence of direct evidence, which it would be extremely difficult to 

 secure, of the destruction of certain specific materials and not of 

 germinal substance in general, it seems more nearly in accord 

 with the general conception of development to believe that Avhat 

 is effected is a disturbance or disarrangement of the constituents 

 of nuclei or cytoplasm, or both, or of the normal relations be- 

 tween and among these materials. Otherwise, and in accord- 

 ance with the latter part of Werber's hypothesis, it is necessary 

 to assume in the germ cell or cleavage group, a whole series of 

 unlike substances whose differentiations are already specific and 

 definitely necessary, not as organizational factors, but as the 

 rudiments or anlagen of the later differentiating tissues and 

 organs of the embryo. It is here that I should take exception 

 to the hypothesis, for there is as yet no direct evidence for the 

 existence, in such early stages, of such differentiated rudiments. 

 It seems much more hkely that it is just the mechanism of dif- 

 ferentiation that is disturbed by these abnormal environments, 

 and not even the earliest formed results of the operation of such 

 a mechanism, .although it is quite possible that there may also 

 be an added actual destruction of specific materials that as such 

 are necessary to normal development. This difference between 

 Werber's view and that stated here is not a minor one; it is the 

 difference between the predeterminational and the epigenetic 

 views of development. 



The fact is an important one, that results exactly parallel to 

 those of Werber (?nd Stcckard) follow upon the mere lowering 

 cf temperature, a condition which eliminates the possibihty of a 



