Cytotaxis 69 



great importance and of far-reaching consequence. Indeed this 

 property has already been recognised and called upon as an 

 explanation of certain developmental phenomena as for instance 

 by Hjalmar Theel who observed that apparent attractions and 

 repulsions occur among the blastomeres of the segmenting egg, 

 Echinocyamus pusillus. 



Zur Strassen especially, has endeavoured to apply the principle 

 of chemotaxis and cytotaxis to the formation of simple epithelia 

 in general and the blastula epithelium of the developing egg of 

 Ascaris in particular. 



He suggests that a simple, single-layered epithelium of cubical 

 or columnar cells, supported by a sub-lying tissue or membrane, 

 may be the result of a chemotactic attraction between the base- 

 ment membrane and the several cells producing a result such as 

 would be produced by placing a pile of shot on a table and then 

 shaking until the shot all settled down into a uniform unilaminar 

 layer. 



For the production of more complicated cases he supposes a 

 much more complicated series of attraction zones, so that one 

 part of a cell attracts its neighbour more strongly than the next 

 part the parts beyond again still more strongly and so on, and 

 thus he accounts for the curves seen in various "free" epithelia. 



When some years ago I originally came to believe in the 

 phenomenon now known as cytotropism or cytotaxis (though 

 perhaps cytokinesis would be a still more appropriate term), 

 I arrived at it as the result of experiments made with a view to 

 determine the cause of invagination such as occurs during the 

 process of gastrulation in Amphioxus. 



The mechanics of the invagination process as seen in Amphioxus 

 during gastrulation have been often discussed, e.g. Hatschek, 

 Goette, zur Strassen, Rhumbler, etc. 



Rhumbler in his recent analysis of the factors involved in the 

 process of gastrulation by invagination distinguishes between the 

 factors which increase invagination when once initiated and those 

 which are the actual cause of it. In his model made of flexible 

 steel rings he perceived that there would have to be an actual 

 force which acts as for instance one's finger acts in producing an 

 invagination in an indiarubber ball, either a pushing from without 



