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The rate of growth of the albino rat.

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From the Laboratory of the Connecticut Agricultural Experiment Station and the
Shefield Laboratory of Physiological Chemistry in Yale University,
New Haven, Connecticut
Growth is commonly measured by the successive changes in
stature or in weight, or by the combination of both of these
measurements. Although each of these methods shows accurately how the animal is growing, none of them is capable of furnishing a direct measure of the rate of growth. This, as Minot2
has pointed out, may be measured by the percentage increase,
based on the initial weight, during a definite length of time, not
by the absolute increments during the same length of time.
Therefore an animal of 50 grams initial weight which gains 5
grams in a given time, has a much higher rate of growth than
the animal of 150 grams which gains 10 grams in the same length
of time. Minot3 has published curves showing the rate of growth
for man, guinea-pigs, rabbits, and chickens, which agree in
exhibiting during the first few days of life a very large daily
percentage increment, which rapidly falls toward the zero line.
These curves indicate that the rate of growth is a variable
function of age, and that the period of life which is ordinarily
associated with the most rapid growth is, in reality, the period of
most rapid decline in the power of growth.
1 The expenses of this investigation were shared by t h e Connecticut Agricultural Experiment Station and the Carnegie Institution of Washington, D. C.,
in connection with t h e nutrition investigations of Dr. Osborne and Dr. Mendel.
Journ. of Physiol., vol. 12, pp. 972 Minot, Senescence and rejuvenation.
153, 1891.
3 Minot., The problem of age, growth and death.
1908. Chap. 3.
This view that the rate of growth is dependent upon age is
supported by P f a ~ n d l e r . ~In a set of curves which he gives of
two infants, one of which was prematurely born, the rate of
growth of the latter was much higher than that of the normal
infant with which it was compared, although its actual increase
in weight during that time was much less. If, however, the
curve is shoved over three months, so as to make it agree with
the age of the normal infant, counting from the time of conception (‘Konzeptionsalter’ as opposed to ‘Geburtsalter’) the two
curves are almost identical. Since the intra-uterine rate of
growth is much higher than the extra-uterine, the abnormally
high rate of growth manifested by this infant at first was due to
the carrying over of the higher intra-uterine rate of growth, corresponding to a greater ‘Lebenspotential’ or ‘Lebenskraft’ than
is normal a t birth. That the cessation of growth is due to age
is a necessary corollary to these facts.
D a ~ e n p o r ton
, ~ the other hand, does not quite agree with this
theory. He says “the reason why the animal ceases a t length
to grow is-not because there is a necessary limit to growth
force at a certain distance from impregnation but because it is
in the nature of the species that the individual should cease to
grow at this point. The indefinite growth of this part, the limited growth of that, are as much group characters as any structural quality.”
The present paper aims to give briefly some data regarding the
rate of growth of the albino rat. The method used is essentially
that employed by iMinot in his study of guinea-pigs. The average growth curve for the albino rat was determined from the
zctual curves of a large number of animals, and from this was
calculated the daily percentage increments for ten-day periods.
Rats raised under identical external conditions exhibit wide variations in weight a t any given age, so that it is difficult t o get a
good average; but as the majority of these animals tended to
4 Pfaundler, Ueber die Behandlung angeborener Lebensschwache.
Med. Kchs., Bd. 54, p. 1417, 1907 (July 16).
5 Davenport, Experimental morphology.
Part 11, Chap. 10, 1899.
I n all the charts the abscissae represent days and the ordinates percentage
increments. I n charts 1, 2, 3, 4 and 5 t h e initial percentage is calculated from
the weight at birth as given by Donaldson: (A comparison of the white r a t with
man in respect t o the growth of the entire body. Boas Memorial Volume, 1906).
Chart 1 Average rate of growth for t h e male albino rat
group themselves about this average curve, it is probably sufficiently accurate for the purpose.
Charts 1 and 2 show the average rate of growth for male and
female albino rats. These are of the same general form as those
of Minot’s for other animals. The very high initial values, 17.7
Chart 2 Average rate of growth for the female albino r a t
per cent for the males, and 15.0 per cent for the females, are due
to the fact that rats, like rabbits, are born in a very immature
state; blind, without teeth or hair, and incapable of any muscular
coordination. In fact, a rat three weeks old is no more mature
than a guinea-pig at birth, from which time on, the curves for
the two animals show more or less agreement. The irregularities in the latter part of the curves are probably caused by an
insufficient number of observations.
Chart 3 shows the rate of growth of a very large rat, whose
curve of growth varied greatly from the average, and chart 4
shows that of a very small rat. These curves have the same gen-
Days 50
Chart 3 Rate of growth of a n unusually large male rat
era1 features-as those for the normal, varying only in the rapidity
of the decline.
Charts 5 and 6 show the rate of growth of two rats which were
stunted from the time of weaning until they were 89 and 314
days old respectively. At this time the food was changed, and
although both of them had reached an age a t which a normal
animal grows very slowly, or not at all, they immediately began
growing a t a rate normal for their size, or even a t a slightly higher
Chart 4 Rate of growth of an unusually small male rat
one, apparently in an endeavor to reach the size which they
would have attained had they not been subjected to a preliminary period of stunting.
Charts 7 and 8 show interesting curves of repair, followed by
late growth. These animals grew normally for a time, then declined seriously and were brought back to a weight normal for
their age, at a rate somewhat above the normal.
These experiments prove conclusively that the power of growth
is not lost so early in life as one would be led to suppose from a
study of the curves of normal animals only. Whether or not
it is ever lost is a question which can only be answered by ex-
10 %
Chart 5 Shows the rate of growth of a female r a t whose growth period was
interrupted b y a period of stunting lasting from 30 t o 89 days of age. When the.
r a t resumed growing, at a n age when growth is normally very slow, i t weighed
37 grams, t h e average weight of a female 30 days old, b u t its rate of growth was
somewhat higher t h a n t h a t of a normal 30-day old female rat.
Chart 6 Shows t h e rate of growth of a female rat which was stunted from time
of weaning until i t was 314 days old. When.the rat resumed growing, a t a n age
at which growth has normally ceased, it weighed 72 grams, the average weight
of a female 50 days old, but its rate of growth was slightly higher than t h a t of a
normal 50 day old rat.
tensive series of experiments in which the animals are stunted
until a very late age before being placed under conditions suitable for growth. The rate of growth is not, however, a function
of age, but of size. Every animal apparently has a certain normal size which it endeavors to reach in spite of vicissitudes during its early life. To what degree it is successful in this endeavor
is probably dependent upon the extent of the injury inflicted
upon it by its early experiences, and the favorableness of the external conditions during its period of growth.
a J L
2I J I I
a J 8
Repair Growth
2 6 Days 300.
Chart 7 Shows the rate
male rat, at an age at which
Chart 8 Shows the rate
male rat, at a n age at which
of decline and repair, followed by late growth, for a
growth has normally ceased.
of decline and repair, followed by late growth, for a
growth has normally ceased.
While this method of measurement is interesting as emphasizing some of the similarities and dissimilarities in the rate of
growth of different types of animals when studied by means of
comparable curves, for ordinary laboratory work it is far inferior to the more commonly used method of determining merely
the successive changes in weight, because even large variations
in weight are entirely concealed. At the age of 260 days the
rat whose rate of growth is shown in chart 4 weighed little more
than one-half as much as the one whose rate of growth is shown
in chart 3-a fact which these curves fail to emphasize. More-
44 1
over, the actual amount of weight regained by the rats whose
curves of repair are shown in charts 7 and 8 was very much larger
than these curves would indicate, because both were large rats
and the percentage increase in weight is therefore much less
striking than the absolute increase.
1. Charts are given showing the rate of growth of albino rats
under normal conditions, and after periods of stunting or of
2. The curves for normal animals are very similar to comparable ones for other types of animals.
3. After prolonged periods of stunting rats are capable of
growing a t a rate normal for their size rather than for their age,
showing that the rate of growth is a function of size rather than
of age as has been commonly supposed.
4. Although this method of representing growth is interesting
for comparative study, for ordinary laboratory work it is inferior
to the usual method of plotting absolute increments, because even
large variations in weight are thereby concealed.
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albina, growth, rate, rat
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