96 



Geometrical relation of Nuclei 



friction may be sufficient to prevent movement due to intercellular 

 attraction. 



If there is an attraction between cell and cell as Roux and 

 others have suggested, that intercellular attraction is a factor 

 which must be taken into account in the study of the mechanics 

 of development. It is probably only in the more delicate tissues 

 of the early stages that the attraction force can have any mechani- 

 cal effect. In later stages and in the more bulky tissues its effect 



Fig. 42. Nine drawings of three cells isolated from a Toad's egg in the gastrulating 

 stage drawn under a camera lucida at intervals of about four minutes as 

 indicated by the figures. 



would be masked by many other factors. It is quite conceivable 

 that contraction, at least in its primitive condition as an attribute 

 of protoplasm and attraction may be phenomena due to the same 

 cause as, for instance, contraction of an envelope like the earth's 

 atmosphere is due to the same cause as attraction of the earth's 

 satellite, namely to gravitation. 



Whether invagination of the Amphioxus gastrula and of other 

 gastrulas of a similar type is due to the existence of an intercellular 



