MORPHOLOGY OF ANIMALS 



and nucleus. They may exist independently as "free cells," 

 or they may be united with other cells, when they are 

 called "fixed cells." They are the smallest units capable of 

 independent existence, the ultimate independent units of 

 organic structure and function. Every cell reproduces by 

 division. The division of the nucleus is typically into 

 equal halves, but that of the cytoplasm may be equal or 

 unequal, — that is the nuclear division is non-differential, 

 but the division of the cytoplasm may be either differen- 

 tial or non-differential. 



(^) Tissues are groups of similar cells together with their 

 products (secretions, etc.). In animals there are two fun- 

 damental types of tissues which give rise to all others; 

 these are epithelia which are composed of cells closely 

 united side by side to form layers, and mesenchyme which 

 consists of cells loosely connected by processes into a more 

 or less spongy mass. In the animal embryo epithelial tissue 

 usually gives rise to the covering and lining layers of the 

 body and the glands, also much of the muscular, nervous 

 and germinal tissues; while mesenchyme produces chiefly 

 connective, skeletal, vascular and fatty tissues. 

 {e) Organs are groups of different tissues, each perform- 

 ing specific functions. Thus the heart is an organ composed 

 chiefly of muscular and connective tissues having the 

 specific function of pumping blood. 



(/) Systems are groups of organs, each performing some 

 general function, such as the heart and vascular system 

 for the circulation of blood, the nervous system for the 

 reception, transmission and coordination of stimuli, etc. 

 {g) Antimeres (homo-typical parts) are bilaterally or 



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