178 A. A. W. lIUr.RECHT. 



the name of alUintois. An older, now more obsolete, sub- 

 division into Achoria and Choriata reveals the presence of a 

 third membrane, the chorion, aljoiit whicli we will have more 

 to say hereafter, and which will explain how this third mem- 

 brane came to fall — so to say — between two stools, when the 

 division into Amniota and Anamnia was established. 



If we now take into account that neither chorion nor 

 amnion nor allantois was ever detected in fishes or in am- 

 phibians, then we must recognise that the problem, how these 

 fostal membranes of the vertebrates did ai-iso, is one well 

 worthy of full consideration. 



Up to now attempts to explain their gradual evolution have 

 utterly failed. So, for example, the suggestion of van Beneden 

 and others that the amnion, as a protective membrane, arose 

 in consequence of the early etnbryo sinking into the yolk-sac, 

 whicii closed up above it, has long since been abandoned. 

 Also Haeckel's idea that the allantois arose by a precocious 

 segregation of the urinary bladder of an early amphibian 

 which took the habit of carrying blood-vessels, at a very 

 eai'ly stage, to the outer wall of the blastocyst, must be 

 dropped by all who object to predestination in evolutionary 

 processes. Whenever an explanation offers itself which does 

 afford a clue to a more logical sequence of events, it should 

 be preferred. 



And turning finally to the outer layer, the chorion, who can 

 be satisfied with the lame explanation that the appearance of 

 this membrane is a necessary sequel to the formation of the 

 amnion, which we find inside of it, and which later, in so 

 many orders of mammals, never even arises by folds, which, 

 however, in their turn are necessary to explain the chorion's 

 appearance ? 



The subsidiarv explanation of all the three embryonic 

 envelopes, whicli I am going to offer you on this occasion, 

 seems to me to have the great advantage of simplifying 

 matters ; especially in this sense, that henceforth we can link 

 them all three to one simpler and earlier stage (which must 

 have preceded in the Carboniferous and in earlier geological 



