30 THE WAY LIFE BEGINS 



to feed it for several weeks after it hatches. Figures 1-5 

 (Plate IV) show the salmon egg with the outer envelop or 

 membrane laid back in order that the embryo may be more 

 readily seen at various stages of development. When hatched, 

 the young wriggle up through the sand of the stream bed. 

 They present a curious appearance (Figures 6 and 7, Plate IV) 

 with their thin, transparent bodies, and big sacs of yolk, 

 which nourish them during the first weeks of growth. This 

 early stage is spent quietly resting on the bottom of the 

 stream, or hiding beneath stones and crevices, out of the 

 way of their many enemies. By the time they are large 

 enough for real activity, the water is warm and their food 

 plentiful. 



It may be the third spring before these fish pass out of 

 the river into the sea. Even then it may take another year 

 before they are grown into full-fledged salmon. 



The Trunk Lines of Life 



The fish stand at the foot of the ladder of back-boned 

 animals. All of these animals, from fish to man, take much 

 the same steps in producing their young. With few excep- 

 tions, fish, frogs, and birds supply their eggs with yolk enough 

 to build the embryo; and sometimes, as with fish and less so 

 with frogs, enough food is also stored to nourish the young 

 for some time after birth. The great majority of the higher 

 forms, the mammals, including man, cannot store enough 

 yolk for such large bodies, and for such long periods. Hence 

 the young, after exhausting the yolk of the egg, and while 

 still in the egg-tube (now called the uterus) of the mother, 

 attach themselves by means of suitable organs and take 

 their building materials from her. 



From fish to man, the back-boned animals resemble each 

 other very closely in their development. These resemblances 

 are seen not only in the growth from egg to adult, but in the 

 parallel way in which the growth changes take place. There 

 are many differences as well as resemblances in their develop- 

 ment, and these make themselves most manifest when the 

 embryo of each animal approaches birth. Indeed the history 



