THE SHAPES AND SIZES OF ANIMALS 145 



small size. The differentiation of parts or organs 

 within a single cell can only proceed up to certain 

 limits, and if of great bulk parts of the cell would 

 be too far removed from the surface to obtain 

 nourishment or to get rid of excretory matter. 

 What the actual limits of size are among unicellular 

 animals we perhaps do not know accurately. 

 Stentor, which may attain a length of $ inch, is 

 usually regarded as one of the largest. Individual 

 cells in higher animals may however attain much 

 larger dimensions : thus, if histologists are right in 

 regarding a single muscle fibre as formed by 

 differentiation within a single cell, then we must 

 grant that a cell may be some inches in length ; 

 while, if the statement be true that a nerve fibre 

 is merely a process of a single cell, it will follow 

 that a single cell may in a man extend from the 

 spinal cord, perhaps from the brain itself, to the 

 extremity of the fingers or toes i.e., may attain a 

 length of some feet, or in an animal the size of a 

 whale as much as 60 feet or more. Such nerve 

 fibres are however exceedingly slender. 



Among the largest cells known to us are the 

 eggs of reptiles, of Elasmobranch fishes, and, 

 largest of all, those of birds. Embryology shows 

 us that the yolk of a bird's egg, which is the only 

 part from which the embryo is directly developed, 

 is morphologically a single cell, and is indeed in 

 the earlier stages of its formation indistinguishable 

 from the ordinary epithelial cells covering the 

 surface of the ovary. The largest eggs are those 

 laid by the Struthious birds, and the yolk of the 



K 



