of Human Monstrosity. 249 



The lungs in all vertebral animals are in tlieir origin probably 

 processes from the mucous layer of the abdominal canal, with 

 corresponding- accompanying portions of the vascular layer, and 

 are in their earliest stage analagous to the swimbladder of fishes, 

 and the lungs of the Proteus and Siren. They are begun in the 

 sixth week, and receive their further development in proportion as 

 the other respiratory apparatus disappear. These latter are the 

 branchial arches, and endochorion of the allanto'is with the pla- 

 centa. These three last respiratory organs do not appear to be 

 intended to perform their functions perfectly at the same time in 

 any class of animals. When the branchial arches of birds and of 

 the mammalia disappear, then the allanto'is is the active respira- 

 tory organ : and where the arches are permanent, as in fishes for 

 the whole of life, and in frogs until the lungs are developed, the 

 allanto'is is not formed at all. 



It is now time that I should declare my opinion respecting 

 the mode in which the appearances, which the subject of this 

 paper presents, have been produced. And this I do with much 

 diffidence, though it has not been hastily, nor carelessly formed. I 

 look upon it, then, as a case of twins, whose germs were originally 

 perfect and distinct, notwithstanding all the difficulties which that 

 opinion includes. The two ova I suppose to have been brought 

 into close contact in the Fallopian tube, and that thus their exo- 

 chorions have coalesced, as is usual, to form a common cortical 

 membrane: and that, besides this, a coalition has also been effected 

 between corresponding points of the germinative membrane, near 

 the disc. The nearly perfect symmetry of the two faces, and of 

 Vol. IV. Pari II. I i 



