1 68 



Eletnentary Zoology. 



of large cells, in the interior of which fibres are being formed. 

 These fibres are seen cut across, so that they, of course, appear 

 as circles. So that what we have in the body- wall of the adult 

 worm is a group of cells of which the boundary lines have 

 disappeared, and of which the protoplasm has been largely 

 converted into long fibres of contractile substance. The nuclei 

 remain unaltered to tell the story of the metamorphosis of these 

 cells. It may be remarked that, though the figure does not 



ONG.MUS. 



Fig. 76.— Section through body-wail of embryo Earthworm. (From Vejdovsky.) 



CUT, cuticle; EPI, epidermis; gl, gland-cell; cir.mus, circular muscles; long.mus, 

 longitudinal muscles ; peri, peritoneum. 



show it, the circular layer of muscles at an earlier stage is 

 developed in precisely the same fashion. 



It is true of the higher animals generally that their tissues, 

 in many cases, depart widely from the typical cellular form ; 

 but it is equally true that those tissues are invariably composed 

 of cells. The most important generalization of histology is 

 that the bodies of all animals and plants are composed of cells. 



We have already several times used the term " tissue." It 

 is now requisite to define this term as used in histology. 



A tissue is a cell aggregate— a group of cells— similar in 

 character, with a function corresponding to their character. 



