No. 398.] SPECIFIC GRAVITY OF ANIMALS. 103 
each other, but the drop in specific gravity began a little earlier 
in their growth in 1899 than in 1898. There is a great differ- 
ence in the two series of frog tadpoles, however, that of 1898 
being much lighter. Even allowing for possible individual 
variation, the discrepancy is still too great to be accounted for 
by the faults of the method. 
In looking back at the history of the two cases I find an 
explanation, which is based upon the work of Galloway ('98), who 
found experimentally that tadpoles of Rana, Amblystoma, and 
Bufo, kept in a warm place, imbibed proportionately more 
water in reaching a certain stage than did those kept in the 
cold. My first series of observations (Table IV) was made on 
eggs of Raza silvatica, which were obtained in cleavage stages 
March 26, 1898. These were kept at room temperature and 
hatched out March 28-31 (two to five days), some reaching the 
length of 7 mm. by the latter date. The second series was 
collected April 11, 1899, and kept out of doors in a shallow basin 
on the ledge of a north window.. On April 22 they had reached 
a length of 7 mm. (after a period of eleven days), although still 
in the egg, and on that day I brought them into the room to 
hasten their hatching. Since the specific gravity of the 
embryos in the second series was throughout so much greater 
than that of the first, I infer that they must have contained 
much less water, therefore have had smaller lymph spaces and 
smaller vacuolated cell regions and water spaces. That is, the 
. embryos reared in a higher temperature must have imbibed more 
water than those exposed to the lower temperature. 
But while the specific gravity depends on the amount of 
water absorbed by the tissues of the embryo, it also depends 
on the amount of animal matter. The proportional amount of 
this animal matter present has been ascertained by weighing, 
desiccating, weighing again, and computing what per cent of 
the first weight the dry weight is. Since the animals from one 
mass of eggs vary individually, I selected them on the basis 
of length, not age, each observation being based on five or ten 
individuals of a given length. . 
In the series of tadpoles of frog and toad I have found the 
Specific gravity, the average fresh weight, the average dry 
