THE OUTER AND INNER TISSUES OF ANIMALS. 303 



wall of the cavity may be again introverted in such way as to 

 intrude into the peri-visceral space. The ccelom thus formed 

 is subsequently shut off. Becoming included among the more 

 truly internal structures, and in part giving origin to certain 

 lining membranes, it has for its chief function the formation 

 of organs for the excretion and emission of nitrogenous waste 

 and of the generative products : some portions of it retaining, 

 as a consequence, indirect connexions with the environment 

 and characters usually accompanying such connexions. 



Here we are not concerned with further details : the aim 

 being simply to indicate the way in which out of the original 

 layer, wholly external, there arise, by primary and secondary 

 introversions, and the formation of intermediate membranes 

 and spaces, the chief contrasts between outer and inner tis- 

 sues, and how there simultaneously go on the differentia- 

 tions accompanying different conditions. 



§ 289a. Another all-important differentiation between 

 outer tissues and inner tissues has now to be set forth — that 

 by which the nervous system becomes established and dis- 

 tinguished. Strangely enough, like the one above described, 

 it is sequent upon an introversion: the nervous system is 

 primarily a skin-structure and develops by the infolding of 

 this skin-structure. 



In creatures possessing the earliest rudiments of nerves 

 these exist in certain superficial cells. Each has a small 

 tubular orifice from which projects a minute hair, and each 

 has on its under side processes running into the tissue 

 below, and serving, as it seems, to conduct impressions from 

 the projecting hair when it is disturbed by contacts with 

 foreign bodies. A plexus of fibres bringing the inner pro- 

 cesses of such cells into communication arises, and forms 

 something like a nervous layer capable of propagating im- 

 pulses in all directions. At a subsequent stage some of the 

 superficial cells, ceasing to be themselves the recipients of 

 external stimuli, sink inwards and become ganglion-cells con- 



