424 SENESCENCE AND REJUVENESCENCE 



Metamorphosis in the amphibia is evidently a process associated 

 with progressive development and physiological senescence, and it 

 may be hastened or delayed by external factors which accelerate or 

 retard development; but the physiological factors immediately 

 concerned in bringing about the changes which occur are still 

 obscure. Metamorphosis unquestionably results in a higher degree 

 of physiological integration, particularly in the higher amphibia, 

 the frogs and toads; in fact, it is in a sense a new integration 

 within the previously existing individual. In the substitution of 

 physiologically younger for older organs and parts, which apparently 

 occurs in amphibian metamorphosis, differences in metabolic rate 

 may play a part, but our knowledge is at present too incomplete 

 to permit definite conclusions. 



EMBRYONIC DEVELOPMENT IN PLANTS 



In most plants embryonic development takes place within 

 special organs of the parent plant, and the embryonic stages are 

 not accessible to physiological investigation as are those of many 

 animals. Moreover, the plant ovum does not in most cases accumu- 

 late a large supply of nutritive substance within its own body, but 

 is nourished by other cells. Only in certain algae and fungi, where 

 embryonic development occurs apart from the parent body, is 

 there any considerable accumulation of nutritive material in the 

 egg itself. 



So far as I am aware, no determinations of oxygen consumption, 

 carbon-dioxide production, or susceptibility have been made upon 

 the embryonic stages of plants, but observation indicates clearly 

 enough that the metabolic changes during these stages are not 

 fundamentally different from those in animals. Fertilization in the 

 plant, as in the animal, initiates an increased activity in the pre- 

 viously quiescent ovum, repeated division occurs with an absolute 

 and relative increase of nuclear substance, and, where nutritive 

 substances are present in the egg, they gradually disappear. As 

 in the animal, the cells resulting from the successive divisions 

 become more or less completely " embryonic" or un differentiated 

 in appearance, and from such cells the new plant individual arises. 

 There is, in short, every visible indication of a process of regression 





