irO DEVELOPMENT OF THE TADPOLE. 



gravity was "900. The well-matured animal, with hind and fore- 

 legs well developed and tail nearly or quite absorbed, had in 

 every case in which an estimation was made, a density distinctly 

 higher than that of water, and approximating to the density of the 

 original embryo. In a series of determinations of the densities of 

 animals in various stages of development, from the time their hind 

 legs began to protrude to the time the tail had become completely 

 absorbed, the following numbers were obtained : — Tail, half 

 absorbed — Density, "956 ; still further absorbed, "962, I'oio, i"oii, 

 i'oi5, i*o2o, i'02i, i*o28, i'o33, 1*034, i"049, i "066. 



In taking these specific gravities, it is most important that the 

 water remains at the same temperature throughout, and that its 

 weight be accurately known, for as the weight of the animal gene- 

 rally varies from i to 5 grains, a very slight variation in the weight 

 of the water in which the experiment is conducted may make a 

 very serious error in the apparent density of the creature. When, 

 however, all jDroper precautions are taken, it is evident, from the 

 figures obtained, that the estimation of the specific gravity of 

 animals in different stages of their development may prove of 

 great assistance in physiological investigations. 



As a special case, may be mentioned its use as an aid in ano- 

 ther portion of the present investigation. Whilst the tadpoles 

 were losing their tails, it was observed that they did not apparently 

 eat, or at least they fed to a much less extent than previously. 

 The balance was called in as an adjunct, with the following 

 results : — The large specimen previously mentioned was weighed 

 almost daily from the time that the tail commenced to be 

 absorbed until the operation was complete, when it was turned 

 out into a field to shift for itself. This process of tail-absorption 

 was accomplished in five days, the weight on July 12, when the 

 absorption commenced, being io'6 grains, with a specific gravity 

 of I '029, and on July 17th, when the tail was entirely gone, the 

 animal weighed only 9*5 grains, with a density exacdy that of 

 water. We thus see that in this case the animal, during the last 

 stage of its metamorphosis from tadpole into frog, lost over 10 

 per cent, of its weight, and at the same time decreased in specific 

 gravity. The reason of these changes in the specific gravity of 

 these animals during different periods of their metamorphoses will 



