surface of the chorion. Bo j anus considered the first of these 

 to be disproyed by his work published in 1815, 



19 



20 



Figures 19 - 21. Illustrations from Bo j anus' work. 



1 9 transverse section of a dog fetus; 

 20 — discorded membrane of a human fetus; 

 21— yolk duct of an adder. 



232 



