13 



that the coatings applied by Mr. Town have not in all instances 

 prevented the permeation of gases. Hence the question remained in 

 much the same state as previous to the researches of Mr. Town, in re- 

 gard to the function of this particular membrane. 



As I had found by frequent observation that almost every tissue in 

 the animal body is characterised by a peculiar arrangement of capil- 

 laries, so uniform in its type as to render it no very difficult matter to 

 determine the nature of the tissue, and hence its function, by the mi- 

 croscopical appearance of the vessels alone ; I was induced to believe 

 that the vascular layer of the allantois, were it a true pulmonary struc- 

 ture, would present that definite character in its capillary vessels that 

 exists in the vesicular lungs of some of the inferior Vertebrata. 



The arrangement, then, in this allantoid membrane, consists of an 

 infinitely minute distribution of capillary vessels of nearly equal size 

 throughout its inner layer, forming an uniform vascular surface cover- 

 ing the large trunks as well as the interspaces of their subdivisions : 

 the anastomoses of the capillaries are so numerous and close, that the 

 areas they leave do not exceed the diameters of the vessels themselves, 

 even when the membrane is somewhat stretched, as in the preparation 

 I shall have the honor to submit to the Society this evening. 



There results from this structure an infinite subdivision of the chan 

 nels of the blood, presenting a most extensive surface in a limited 

 space, to the action of the air, which permeates the shell and its pro- 

 per membrane. 



If the vascularity of this membrane be compared with the lung of 

 the frog, salamander or serpent, the similarity of the type will be at 

 once evident, and carries with it a strong corroboration of the previous 

 theory of the respiratory character of the allantois in birds. It must 

 be remarked, however, that as the blood-corpuscules of the Reptilia are 

 much larger than those of the bird, so the capillary vessels of their 

 lungs are of much greater size than in the specimen of allantois brought 

 to the Society to-night ; the difference between them being one of 

 proportion, not of type. 



The other preparations explain the character of the vascularity of 

 the vitelline membrane at the 1 9th day of artificial incubation. 



Immediately surrounding the remains of the vitelline area are seen 

 on the internal surface of the yolk-sac, the commencement of a series 

 of radiating folds, which dip deeper and deeper into the interior of the 

 sac, presenting an elegant series of undulating plicae, which increase 

 in depth as they advance, and separate more and more from each 

 other. These folds appear at first sight to consist of a simple granu- 



