16 THEORY OF EVOLUTION 



It was recognized, of course, that many em- 

 bryonic stages could not possibly represent 

 ancestral animals. A young fish with a huge 

 yolk sac attached (fig. 6) could scarcely ever 

 liave led a happy, free life as an adult individ- 



FiG. 6. Young trout (Trutta fario) six days after hatching. 

 (After Ziegler.) 



ual. Such stages were interpreted, however, 

 as embryonic additions to the original ancestral 

 type. Tlie embryo had done something on its 

 own account. 



In some animals the young have structures 

 that attach them to the mother, as does the 

 placenta of the mammals. In other cases the 

 young develop membranes about themselves — 

 like the amnion of the chick (fig. 7) and 

 mammal — that would have shut off an adult 

 animal from all intercourse with the outside 



