I-J. ^'liii^^O^ DEVELOPMENT AND META- 



..jy|,:^|i%^"^^'^? MORPHOSIS 



CHAPTER II 





--V. I HAT animals are born or hatch from eggs in 

 \J an immature condition is such familiar natural 

 history that we are likely to overlook the 

 signiticance and consequences of the fact unless 

 our attention is particularly called to them. 

 This condition of immaturity makes it necessary 

 that part of the free life of the organism has 

 to be devoted to growth and development and 

 has to be undergone in an imperfect condition, 

 a condition of structure and physiology, indeed, which may be very different 

 from that of the parents or of maturity. While most animals that are born 

 alive resemble the parents in most respects, always e.xcepting that of size, 

 many of those animals which hatch from eggs deposited outside the body 

 of the mother issue from the egg with few indeed of the characteristics of the 

 parents and may be so dissimilar from them that only our knowledge of 

 the life-history of the animal enables us to recognize these young individuals 

 as of the same species as the parent. The butterfly hatching as the worm- 

 like caterpillar, and the frog as the fish-like tadpole, are the classic e.xamples 

 of this phenomenon. The mammals, our most familiar examples of animals 

 which give birth to their young alive and free, nourish, for weeks or months 

 before birth, the developing growing young. But with egg-laying animals 

 usually only such nourishment is furnished the young as can be enclosed 

 as food-yolk within the egg-shell. As a matter of fact, some young which 

 hatch from eggs, as, for example, chickens, quail, etc., hatch in well- 

 developed condition; and some young mammals, nourished by the mother's 

 body until birth, are in a conspicuously undeveloped state, as a young 

 kangaroo or opossum. But nevertheless it is generally true that an animal 

 hatched from an egg has still a larger amount of development to undergo 

 before it comes to the stature and capacity of its parents than one which is 



35 



