304 PHYSIOLOGICAL DEVELOPMENT. 



nected with the nervous plexus agents, as we must suppose, 

 for the reception, multiplication, and diffusion of the impulses 

 received from the outer cells. 



As thus far developed, the nervous structure is one fitted 

 only for a vague stimulation of dispersed contractile fibres, 

 causing movements of an undirected kind. A concentration 

 of these superficial nervous structures is a probable prelim- 

 inary to the next change an all-important change. For a 

 part of the surface begins to sink inwards, forming, in the 

 Vertebrata, a groove ; and from the lining cells of this groove, 

 which presently closes over, the central parts of the nervous 

 system arise: definite nerves having meantime, as we may 

 suppose, been developed out of the indefinite nervous plexus. 



Neglecting what there is in this of a speculative nature, it 

 is sufficient for the present purpose to recognize the un- 

 doubted fact that the nervous system is developed from the 

 ectoderm, and that, originally external, it is made internal by 

 a process of sinking in or by a process of definite introversion. 



290. Whether direct equilibration or indirect equilibra- 

 tion has had the greater share in producing these fundamental 

 contrasts between the inner and outer tissues of animals, 

 must be left undecided. The two causes have all along co- 

 operated modification of the individual accumulated by 

 inheritance predominating in some cases, and in other cases 

 modification of the race by survival of the incidentally fittest. 

 On the other hand, the action of the medium on the organism 

 cannot fail to change its surface more than its centre, and so 

 differentiate the two; while, on the other hand, the surfaces 

 of organisms inhabiting the same medium display extreme 

 unlikenesses which cannot be due to the immediate actions 

 of their medium. Let us dwell a moment on the antithesis. 



We have abundant evidence that animal protoplasm is 

 rapidly modified by light, heat, air, water, and the salts 

 contained in water coagulated, turned from soluble into in- 

 soluble, partially changed into isomeric compounds, or other- 



