1G2 



MEMOIR ON THE REPRODUCTION OF THE OPOSSUM. 



healthy, warm-blooded mammifer can possibly be developed but by means of a placental 

 or placentoidal apparatus — and I repeat that the absence of all traces of it in my speci- 

 men ought not to disturb my faith in trie invariability of the genetico-generic laws. A 

 mammal is, ipso facto, a placental animal. 



Having the animal well secured in a dorsal decubitus, I opened the abdomen with a 

 dissecting needle, taking care not to wound any of the interior structures. There was 

 much fat under the dermal surface. The skin was slightly adherent. The tela cellulosa 

 is extremely delicate in these early stages of existence. I found that the muscles of the 

 tongue and those of the abdomen were readily displaced out of their cellular sheaths, by 

 raising and removing them with the point of a dissecting needle. 



I opened the thorax with a delicate scissors, and found the sternum and the ribs very 

 stiff and elastic, cutting like cartilage. I presume no phosphate was as yet deposited, at 

 least in those organs. 



The sides of the abdomen and»<thorax being cut away, aljowed me to inspect, with the 

 lens, a most beautiful and perfect set of organs, as the heart, with its pericardium; the 

 liver; a very large and loaded stomach; and a considerable tractus of intestines, all filled 

 and plump with fulness. 



I was much struck with the appearance of the lungs. On each side of the heart I 

 discerned a number of bubbles, which, on closer investigation, proved to be spherical 

 air-vessels, or pulmonary vesicles full of air. They rather resembled a bunch of trans- 

 parent hydatids or grapes, or a collection of fine soap bubbles. I have given a mag- 

 nified view of these vesicular lungs in the last figure. 



I removed some of those of the left side, and placing them on my microscope, viewed 

 them with a power of one hundred and fifty. They were pressed beneath a plate of 

 mica. The figure shows the microscopic appearance of the compressed air-cells. The 

 intervesicular tissue was manifestly organized, and of a brownish tint; not a mere sarcode. 

 I did not see any blood-vessels. I presume the blood had escaped, for I had taken it 

 up with points of blotting-paper, out of the thoracic and abdominal cavities, in order to 

 have a clearer view. 



The lungs were very large; and the quantity of 

 air contained within them must have been vast, as 

 compared with the wants of aeration for the few 

 drops of the creature's blood. 



I am not aware that Mr. Owen and Mr. Ed- 

 ward Qucckett have as yet resolved the question 

 whether the swimming bladder of fishes is also a 

 true organ of aeration. Mr. Queckett was making 

 researches on that point in 1845, when I had the 

 pleasure to see him in London; and I sent to him 

 some specimens of the gar-fish of the western 

 rivers, with a view to his inquiries on that very 

 head. If those naturalists have discovered that the swimming bladder is an organ of 

 aeration, then I cannot but think that my marsupial didelph is as amply supplied with 



