DEVELOPMENT OF THE TADPOLE. 09 



The embryo in the stage represented by Plate II. had a spe- 

 cific gravity of 1-027, but owing to the absorption of the segmenta- 

 tion cavity, this had increased in five days to 1 '068, which was the 

 highest density observed, and from this time the density gradually 

 diminished. Thus, on March 7th, the density being i-o68, it was 

 I '06 1 2 on March loth, and 1-062 on the 12th of the same month. 

 These weighings were made on embryos developing from spawn 

 laid on March 2nd, 1885, and were in about the same stage of 

 development as those figured in Plate VIII., Vol. I., as they had 

 just emerged from their encircling envelope, and were able to 

 commence using their tails whilst, holding on by means of the 

 claspers, to the sides of the aquarium or to the envelope from 

 which they had escaped. The remainder of the specific-gravity 

 estimations were not undertaken till two years after — namely, in 

 the spring of 1887. 



From the loth of March the specific gravity went on slowly 

 decreasing, till on June 22, when the tadpole was at its highest 

 stage of development as a tadpole, and before its legs had com- 

 menced to protrude, the specific gravity had sunk to -774. The 

 weight of the animal thus experimented on in air was 3-6 grains, 

 and the weight of water equal in bulk to itself was 4-65 grains, 

 thus giving its weight as -774 as compared with the weight of an 

 equal bulk of water. 



We thus find that at this stage of its development the animal 

 is decidedly lighter than water, and this is apparently due to the 

 power of taking in considerable quantities of air, which may be 

 expelled after having been used for respiration ; and I believe, 

 though I am not positive on the point, when the animal wishes to 

 sink and remain at the bottom, a portion of this air is condensed 

 and its place supplied by water. 



Continuing our investigations on these specific-gravity experi- 

 ments, we find that as the legs grow and the tadpole merges gra- 

 dually into the state of the perfect animal, it becomes again 

 comparatively heavier, its density becoming as nearly as possible 

 that of water. Thus, the same animal, with its hind legs just 

 appearing through the skin, weighed 4-1 grains, and had a density 

 of -752 ; with hind legs well developed, it weighed 5 grains; with 

 a density of -869 ; and when the forelegs had also protruded, the 



