70 Geometrical relation of Nuclei 



or a pulling from within which initiates the invagination. Once 

 started, then lateral pressure caused by the multiplication of 

 cell units will increase it; but without forces other than those 

 which he supposes to exist, lateral pressure cannot initiate 

 invagination. 



It has been suggested that invagination of a gastrula such as 

 that of Amphioxus may be brought about by the absorption of 

 the fluid within the blastocoel, causing the collapse of one side. 

 Hatschek suggested this as a possible cause in Amphioxus ; but 

 apart from the unlikelihood of the thicker side being the invagi- 

 nated side as is actually the case in Amphioxus, if this were the 

 cause, it has been completely put out of question by the fact 

 noted by Morgan and Hazen that invagination will occur when 

 a small aperture exists in the blastula wall. 



Rhumbler like Goette points out that the shape assumed by 

 cells forming the wall of a blastula is conical, with the smaller end 

 pointing inwards, but that the cells of the invaginated side of a 

 gastrula are conical with the smaller ends pointing towards the 

 original outside, and Rhumbler suggests that a change in shape of 

 the cell has been the cause of the invagination. Surely this is con- 

 fusion of effect and cause. The living cell in an undifferentiated 

 state is an almost perfectly fluid body and the rectangular or 

 hexagonal shape assumed in early undifferentiated tissues is much 

 more easily to be explained as the result of environmental pressure 

 than as being due to its own intrinsic properties. And does not 

 a cell isolated from blastula or gastrula tend to assume a spherical 

 form? 



He argues, however, that the change in shape is due to the 

 absorption of fluid by the inner portions of the cells causing 

 thereby a bulging of their inner surfaces. This, occurring in many 

 or most of the cells of one part, would, he says, produce the change 

 in shape, and together with the lateral pressure due to multiplica- 

 tion of the ectoderm units would bring about invagination. 



Przibram commenting on this and other suggestions offered 

 concludes " Blastula tion and gastrulation depend on chemotactic 

 effects started by processes of assimilation, which not only cause 

 passive mechanical displacements, but also active migrations of 

 cells." 



