26 EVOLUTION AND GENETICS 



type. The embryo bad done sometbing on its own 

 account. 



In some animals tbe young bave structures tbat 

 attacb tbem to tbe motber, as does tbe placenta of tbe 



Fig, 10. — Young trout, six days after hatching. 

 {After Zieyler.) 



mammals. In otber cases tbe ^^oung develop mem- 

 branes about tbemselves — like tlie amnion of tbe 

 cbick {fig. 11) and mammal — tbat would bave sbut 

 off an adult animal from all intercourse witb tbe out- 



FiG. 11. — Diagram of chick, show- 

 ing relations of amnion, allantois, 

 and yolk. {After Wilder.) 



