16 THEORY OF E\OLUTIO\ 



It was recognized, of course, that many eni- 

 Inyonic stages could not possibly represent 

 ancestral animals. A young fish with a huge 

 yolk sac attached (tig. 6) could scai'cely ever 

 Jiave led a hapjjy, free life as an adult individ- 



Fic. (i. Yoiiiif;- triiut ('rriitta fariii) six cl;iy,s iiricr lialiiiiiii! 

 (After Zifg'ler.) 



ual. Such stages were interpreted, however, 

 as e)nbrijo)iic additions to tlie original ancestral 

 type. The embryo had done sometliing on its 

 own account. 



In some animals the voung have structures 

 that attacli them to the niothei', as does the 

 placenta of tlie mammals. In otiier cases the 

 young develop meml)ranes about themselves — 

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

 mammal — that would Jiave shut off an adult 

 animal from all intercourse with the outside 



