14 HIGH SCHOOL ZOOLOGY. 



7. Animal cells have the same component parts as vegetable cells, 

 i. c, they are formed of a protoplasmic cell-body containing a nucleus 

 and limited (frequently) by a membrane or wall. The latter, wliich 

 plays so important a pirt in tlie suppoit of tlie plant, resigns this func- 

 tion in the higher animals to the intercellular substance, which, although 

 like the cell-wall formed by the activity of the protoplasm, difiers there- 

 from in rarely exhil)iting the territories belonging to the constituent 

 ci-Us. All the cells of the animal body are, like those of the plant, de- 

 rived from the division (and the differentiation of the products of the 

 division) of one cell — the egg-cell, and the first results of such division 

 and differentiation are the formation of embryonic layers somewhat 

 analogous to the primary meristems of the plant-embryo. Perhaps the 

 most characteristic difference between the plant and animal embryo is 

 that in the latter some of the most important organs are developed Ijy 

 the infolding of the originally superficial epithelial layer. 



8. The four categories under which animal tissues fall may shortly be 

 characterized as follows : — (Fig. 3.) 



I. Epithelial Tissue is that which is disposed in the form of one 

 or more layers of distinct cells on the free surfaces of the body, including 

 the alimentary canal, the lining of the ccelom, the cavities of the nervous 

 system, etc. The cells may l)e cylindrical, columnar, cubical or scale- 

 like in form, their free surfaces may l)e covered with a resistant cuticle, 

 or provided with delicate continuations of the protoplasm in the form fif 

 cilia or hairs. If their duty is to receive impressions and transmit 

 them to nerves, they constitute neuro-epithellum ; if they secrete 

 some characteristic product they constitute glandular epithelium, 

 and are generally turned in from the free surface for protection ; if they 

 serve merely to form hard structures for protection of underlying parts 

 or for defence, they are modified into horny epithelial scales, feathers, 

 hairs, hoofs, nails, horns, etc., while if 'they are converted into eggs, 

 etc., they constitute germinal epithelium. 



II. Connective Tissues- — These constitute the framework of the 

 body, which in some organs is of the utmost delicacy, in others, the true 

 skeletal tissues, attains great firmness and hardness. Sometimes the 

 cellular elements are distinct, in which case they may be free to wander 

 through the interspaces of the tissues in the form of amoeboid or 

 wandering cells, or be more limited in their mobility like the pigment- 

 cells; or be fixed and flat like epitlieliuni, or glol)uIar and filled with fat, 

 or branched and communicating with their neighbours. Sometimes in 

 the adult tissue the protoplasm is almost all converted into intercellular 



