External Factors that Influence Growth 255 
continue to supply nourishment to the young in the uterus; and 
while the embryos might not grow as fast as do those in a well- 
nourished individual, they would probably grow at least as long 
as reserve material was present in the mother, and perhapg even 
while the formed tissues of the mother were slowly being used up. 
It would be interesting to carry out a series of exact experiments 
of this kind on such animals as rats, mice, or dogs — especially 
on forms that can go a long time without food. 
These conditions show plainly that growth depends on other 
factors as well as on the amount of food that an animal obtains, 
or has in reserve. 
Not only the amount of food but the kind of food also has 
an important bearing on the rate of growth. Yung fed tadpoles 
on several kinds of food, including aquatic plants, the jelly of 
frogs’ eggs, yolk of hens’ eggs, white of hens’ eggs, fish, and beef. 
The results are shown in the following table: — 
Aquatic | JELLY oF YELLow oF} WHITE oF 
Prants | Frocs’ Eccs| Hens’ Eccs| Hens’ Ecos| FISH | Beer 
Length of tadpole 18.3 23.2 26.0 33.0 38.0] 43.5 
Breadth of tadpole 4.2 5.0 5.8 6.6 8.8] 9.2 
Kinp oF Foop 
Fed on beef the tadpoles grew nearly three times as fast as on 
plant food. The experiments were continued, in some cases, 
to the time of metamorphosis. The large tadpoles transformed 
first, but the smaller ones not until they had grown large. 
Cuénot has also reared tadpoles under different conditions, and 
has recorded the time of their metamorphosis. He also found 
that the well-fed animals transformed first. Barfurth has shown, 
on the other hand, that starvation just prior to the time of meta- 
morphosis hastens the change. 
Experiments made by Lawes and Gilbert have shown that 
‘“‘srowth tends to increase with the quantity of nitrogeneous 
food.” A curious instance confirmatory of this conclusion is 
given by Présher. He compared the times required by different 
mammals to double their birth rate with the composition of the 
milk of the mother. His results are given in the following table. 
