Experiments on Frogs' eggs 93 



pulled asunder. So far as I know invagination to form a gastrula is 

 never spasmodic. There are never any perceptible contractile 

 movements. On the other hand, the attraction hypothesis requires 

 no special teleological developments but presupposes a constant 

 and universal principle which, guided by other factors, in this case 

 largely inanimate, leads inevitably under normal conditions to a 

 certain definite result. 



With a view to determining the nature of the alleged attractions 

 between cell and cell such as have been stated to occur between 

 the isolated blastomeres of gastrulating frogs' eggs, I repeated 

 some of Roux's experiments by isolating the blastomeres of Rana 

 temporaries and Bufo vulgaris and observing any changes in position 

 or shape which take place when such cells are floating in ordinary 

 water, or in a mixture of salt solution and albumen as used by 

 Roux. On the whole I found the attraction to be less noticeable 

 than I had expected. 



In many cases, that is to say between many pairs of cells which 

 were closely situated to each other, there was no movement at all. 

 This Roux also found. But on the other hand certain cells origin- 

 ally a short distance apart from one another perhaps half a cell 

 diameter certainly approached one another and came actually in 

 contact; but I did not see any that became pressed together so 

 as to be flattened, indicating any considerable strength of attrac- 

 tion force. 



I experimented on frogs' eggs which were laid on the 27th March, 

 and on toads' eggs a fortnight or so later in April. 



They had been some time in dishes on my balcony, and were 

 in the gastrulating stage, the lower lip of the blastopore was just 

 about to form. I broke the eggs and gently teased the cells apart, 

 some in the water in which they were and some in a mixture of 

 | per cent, salt solution and filtered white of egg after Roux's 

 instructions. 



With the former I was much more successful in observing 

 attraction movements, with the latter I was more successful in 

 seeing pseudopodial movements. 



After treatment of an egg as described a large number of the 

 cells are isolated, though many pieces of gastrula wall remain 

 unteased. 



