688 THE MICROSCOPE. 



mouth, and the crouching attitude of the all but perfect 

 reptile. 



" By the process now described, the three systemic 

 arteries become continuous with the corresponding efferent 

 trunks that convey the blood for distribution through the 

 body, while, simultaneously, the vital fluid is being 

 abstracted from the special trunks belonging to the gill 

 and its vascular crests. These, with the gill structure 

 connected with and dependent upon them, being thus 

 deprived of their blood, shrink, become absorbed, and 

 disappear. Such appears to be the beautifully simple 

 mechanism by which the transition in the type of the 

 respiratory function from fish to reptile is accomplished. 

 If we take a tadpole a few days old, when the outer gills 

 are fully developed, and immerse it for another few days 

 in a weak solution, of chromic acid (Mr. Archer's method), 

 we may, by placing the tadpole under a dissecting micro- 

 scope, and with the aid of a needle and camel-hair brush, 

 then remove the integuments, disclose the tufts of the 

 inner gills, and by carefully getting rid of a prominent roll 

 of intestine that occupies the upper part of the abdomen, 

 succeed in revealing the incipient lungs. These are situated 

 behind the gut and close to the spine, and appear as a pair 

 of minute tubular sacs, united at their upper and open 

 extremities. The chromic acid renders the tissues friable, 

 so that they can be readily peeled away." 



That we may see how the circulation is carried on during 

 life in the gills, the outer covering must be carefully raised, 

 or even stripped off. This can be accomplished by put- 

 ting the Tadpole under the influence of chloroform a 

 drop of the fluid is sufficient for the purpose. 1 



The blood-vessels of mammals are divided with refer- 

 ence to their structure into arteries, capillaries, and veins ; 

 yet these three divisions are by no means broadly separated 

 from each other, inasmuch as the capillaries are continued 

 into the veins just as imperceptibly as they arise from the 

 arteries. But with reference to their structure, while the 

 capillaries possess only a single coat without any special 

 structure, the larger vessels present, with but few excep- 



(1) We refer the reader to Mr. W U. Whitney's highly interesting and valuable 

 paper iu the Trans. Micros. Soc. for 1861 and 1867. 



