82 Geometrical relation of Nuclei 



apart from any exact or near similarity existing between the sup- 

 posed forces. 



The explanation of invagination during gastrulation in Amphi- 

 oxus is put forward as a little further evidence in favour of the 

 existence of some force which acts like gravitation or magnetism 

 or statical electricity, and can be described as acting from a centre, 

 but which is probably of a different nature and with different laws, 

 and is an attribute of living matter alone. 



For it has been made clear, I hope, that Amphioxus in its 

 process of gastrulation fulfils all the conditions required by the 

 model of indiarubber balls, provided it can be conceded that 

 (1) there is an attractive force between cell and cell, "acting from 

 a centre"; (2) that the nucleus approximately indicates the 

 position of the centre from which this force may be described as 

 acting; (3) invagination can only take place after a certain size 

 relative to the size of the unsegmented ovum, the distribution of 

 yolk, and degree of force of attraction has been attained by the 

 blastula. 



By reason of the original structure of the ovum the centres of 

 attraction take up their required position whereby invagination 

 must ultimately occur. So long as the essential structure is not 

 interfered with, the ovum may be subdivided without destroying 

 the power to invaginate. 



A gastrula ^ the normal size may be obtained so long as the 

 subdivision of the ovum has been always by a vertical plane. But 

 the gastrula stage is not reached if the division has been made 

 horizontally. 



This fact receives satisfactory explanation on the hypothesis 

 enunciated above. 



The attraction of daughter cells immediately following separa- 

 tion is of course very marvellous ; because whatever may be the 

 details and however complicated may be the mechanism of cell 

 division, yet the process of cell division reduced to its simplest 

 expression must imply the origin of bipolarity or a bicentred con- 

 dition involving an antagonism or repulsion between two centres 

 within a body which at one time had been single centred. That 

 is to say, the incipient daughter nuclei (and centrosome of course) 

 while still forming parts of their disappearing parent seem to repel 



