342 Elementary Biology. 



Growth, in the animal as in the plant is due to the excess 

 .of repair over waste, of anabolism over katabolism. This 

 anabolic excess is proportionally very great in the young, 

 becoming less as maturity is approached. In the animal 

 this early period may be divided into an embryonic stage 

 spent inside the egg-membrane, and a postembryonic period, 

 during which, as in the frog, metamorphosis may occur. 

 Growth in the lower animals especially is frequently accom- 

 panied by ecdysis, or skin-casting, when the skin, whether 

 soft as in the frog, or stiffened by a deposit of keratin (insects), 

 or carbonate of lime (crustacea), is cast off and a new one is 

 assumed. Ecdyses are concomitant with periods of more 

 active growth. 



Motion. Motion is brought about by the contraction 

 of muscle-cells in one direction ; it is therefore definite. 

 Indefinite motion in the higher animals is still represented 

 by the amoeboid movements of white blood-corpuscles and 

 lymph-cells, c. 



A muscle contracts in response to a stimulus, usually 

 nervous. Muscles are themselves irritable, for if the peri- 

 pheral terminations of nerves be poisoned by curara, the 

 muscles are still able to respond to a non-nervous stimulus, 

 such as a prick from a needle or the application of certain 

 chemical compounds, for example acetic acid. As we have 

 already seen, the muscles of the body may be arranged in 

 two categories : muscles which are intrinsically parts of 

 organs, e.g. the muscles of the heart, alimentary canal, &c. ; 

 and muscles which are attached to hard parts, by the con- 

 traction of which motion of the limbs and trunk, or of the 

 body as a whole, is effected. A single muscular contraction 

 may be represented by a curve rapidly attaining a maximum, 

 and more slowly regaining the abscissa. If, however, the 

 muscular contraction be followed immediately by another 

 before the first contraction has reached its maximum point, 

 the total maximum contraction is considerably greater than 

 the maximum which would have resulted from one stimulus 



