vii EVOLUTION OF SENSE-CELLS 331 



Even the intestinal musculature of the Arthropods is trans- 

 versely striated ; and the intestine obviously in Crustacea as 

 well as in Insecta has in a pre-eminent degree a respiratory 

 function, and on account of this, as well as in many cases in 

 consequence of the ceaseless action of the digestive machinery, 

 it is in constant movement. 



But if we examine the muscles of the thorax of flies in 

 the beginning of spring when they have just been revived 

 by the warmth, and have not yet flown, we find that the 

 fibrils are almost all smooth without any striation, an 

 observation which has been for years regularly repeated in 

 my histological demonstrations. This spring in these classes 

 we made the experiment of repeatedly shaking up the flies 

 so as to make them move and fly, and thought, even after 

 this slight motion of the wings, we could notice that a much 

 greater number of the fibrils were striated than before. 

 When we examined the thoracic muscles of flies this summer 

 we found all the fibrils most perfectly striated. 



EVOLUTION OF THE VARIOUS SENSE-CELLS FROM A COMMON 

 CELL LAYER 



The history of the nervous system proves if possible still 

 more conclusively than the origin and development of muscle 

 how completely the origin and elaboration of organs are 

 governed by external stimuli and by exercise. 



It is actually required by my theory that the nervous 

 system should, as it in fact does, proceed from the epiblast. 

 For the nervous system consists of the organs of relation, that 

 is, the organs by which the body is placed in relation to the 

 environment, by the reception of stimuli and the excitement 

 of a response to them. If, as we must assume, not only 

 according to the biogenetic law, on account of all the facts 

 of embryogeny, but also on account of the structure of the 



