STRUCTURAL CHARACTERS OF ANIMAL ORGANISMS. 



47 



connection with nerves, and aid in forming the special nerve 

 terminals.* 



Nerve Tissue. The great nervous centres are formed from 

 certain cells of the outer germinal layer, which, in the earliest 

 days of the embryo, dip in as a furrow, and are gradually cut 

 off from the parent tissue by the rapid growth of the middle germ- 

 layer. In looking for special conducting tissue in animals pos- 

 sessing the most simple structure, we find cells which would seem 

 to possess certainly a twofold, and possibly a threefold, function 

 one of which is conduction. In the so-called " neuro-muscu- 

 lar" cells of the hydra, processes are described as found to pass 

 off from them, and to unite beneath the ectoderm with other fibre- 

 like processes, which are eminently contractile. Here we find 



FIG. 16. 



c- 



* <* 



FIG. 15. Epithelial cells, some of which are filled with mucus (d), forming goblet-like 

 cells. (Cadiat.) 

 FIG. 16. Neuro-muscular cells of hydra. TO. Contractile fibres. (Kleinenberg.) 



for the first time a portion of protoplasm specially devoted to 

 acting as a conductor of impulses, and attached by the one end 

 to a contractile fibre, and by the other to a surface (sensory) cell. 

 The intimate relation between the development of nerve and 

 muscle fibres is thus established, and we have the first attempt at 

 a nerve mechanism, viz., a cell capable of receiving impressions, 

 and a fibre capable of transmitting the results of these stimuli. 

 As further differentiation proceeds, each of these parts becomes 

 more distinct from the other, and ultimately the adult nerve tis- 

 sue is found to be made up of nerve or ganglion cells, nerve fibres, 

 and special nerve endings. The fibres commonly act as lines of 



* A fuller account of the Histology of these tissues will be found in the 

 chapters specially devoted to these subjects. 



