PHYLOGENY AND ONTOGENY OF NEURONA 43 



blood for its affinity with oxygen, and nerve-cells, 

 like the acoustic and optic, which have specific 

 affinities with sound and light. The epithelial cells 

 which we have seen forming the external covering 

 of the gastrula are more differentiated in the 

 actiniaries (fig. 2). The incitations, or rhythms, of 

 which we have spoken, and which were received 

 and stored in the external cells of the gastrula, are 

 those which give rise to the cells called mio- 

 epithelial, because, as they register sensations 

 through their external extremity, they are the 

 origin of sensation, and oblige the opposite ex- 

 tremity of the cell to transform this sensation into 

 action, making it contractible. Therefore sensibility 

 and mobility seem united in one and the same 

 element, and hence the name of mio-epithelial cell. 



Referring to the ectoderm of the gastrula in 

 fig. 3, A represents a portion of the epithelium of 

 an actinia, where one sees how one of the ecto- 

 dermic cells (ri) is differentiated from its fellows, 

 and the nerve-branches which start from the body 

 of the cell are also seen (n). 



The other cells, less prone to register the specific 

 incitations of the external agents, retain their 

 epithelial form and serve as a support and protec- 

 tion. This cell is called neuro-epithelial. 



Fig. SB represents one of these cells, which have 

 advanced to a higher grade of differentiation. 



