In a paper on "The Influence of Diet on Growth and Nutrition," in the Journal of Physiology (vol. xxxiv., p. 3), Dr Chalmers Watson showed that in rats a diet of ox-flesh begun when the animals were weaned interfered with the development of pregnancy, none of the four flesh-fed animals having young, whereas the control animals from the same litter all became pregnant. On the other hand, it is stated of three families fed on horse-flesh from the age of two and a half months approximately, that all became pregnant, from which he concludes that "the use of this diet in animals of this age appears not to affect the supervention of pregnancy".

It was further found in the case of the meat-fed animals which became pregnant and suckled their young, that the mammary tissue was less developed than in the control bread-and-milk-fed rats.

1 B. P. Watson, Journal of Physiology, vol. xxxiv., 1906.

2 B. P. Watson, Proceedings of the Royal Society of Edinburgh, vol. xxvii., 1906-7.

At I)r Chalmers Watson's suggestion I have extended these observations, and with a larger amount of material at my disposal am able to amplify his statements, and as regards the question of fertility slightly to modify them.

The method of conducting the investigation was as follows: - Twelve female rats and several males were put on a bread-and-milk diet, and the females were continued on this throughout pregnancy and lactation. These served as the controls. Seventeen females and five males were put upon an ox-flesh diet, but were otherwise under exactly the same conditions as the bread-and-milk animals. The animals were begun on the meat diet at various ages, from the second up to the fourth month, and some of them were kept on the diet for as long as five months.

1. Effect Of A Meat Diet On Fertility

Of the seventeen animals fed upon a meat diet only eight became pregnant, and of these four bore young within twenty-one days - the usual gestation period in the rat - of being put on the diet, so that only four actually conceived while on the diet. Of these latter one had been twenty-four, one twenty-five, one twenty-seven, and one thirty days on an exclusive ox-flesh regimen.

The other nine animals, although kept for several months, did not conceive, and this in spite of the fact that they were seen to copulate freely right up to the end of the experiment. This reservation must, however, be made, that one of these nine animals probably had young which were eaten, and it is just possible that this happened in other cases. It is not at all probable, however, as the animals were frequently carefully examined, and if there was any indication of their being pregnant, were at once transferred to separate cages.

Of the twelve animals fed on a bread-and-milk diet, all became pregnant and had young, so that we may conclude that a meat diet is decidedly prejudicial to the occurrence of pregnancy in rats when the diet is begun when the animals are from two to four months old.

In order to determine whether the fault resided in both sexes or in only one, a fresh male which had been fed on bread and milk was put beside the sterile females which had been on meat for several months. When the animals were killed, some time after, one of them was found to be in an early stage of pregnancy, and must have been impregnated by the bread-and-milk male.

This would appear to indicate that the cause of the sterility is partly due to the male, but we have not had sufficient material to form any more definite conclusion regarding this.

2. Effect Of A Meat Diet On Lactation

For this part of the investigation the same animals were used, viz., the twelve controls fed on bread and milk, and the eight meat-fed animals which became pregnant. The point specially attended to was the weight of the mammary tissue of the animals killed after suckling their young for varying periods.

In the nursing rat the mammary tissue forms a continuous sheet spread under the skin of the abdomen on each side of the middle line. In addition there are extensions of it into the axillae, up along the neck and into the groins, while in some cases it spreads out so much laterally most to reach the hack. The nipples are in a douhle row extending from thorax to the groins.

The animals were killed at different periods during lactation. The weight of the mother and the number and weight of the young at the time of death were ascertained. The skin and subcutaneous tissue of the mother's abdomen was immediately removed down to the muscle, care being taken that no mammary tissue was left behind in the axillae or in the groins. The skin was put into 5 per cent, formalin for a few days, when it was quite easy to separate the mammary tissue from the skin on one hand and from the areolar tissue on the other. If an attempt were made to strip the mamma? before first fixing in formalin, it was found that a great deal of milk was squeezed out. The immersion in formalin prevented this, although it extracted a small quantity of milk, as shown by its cloudy appearance at the end of twelve hours. It is to be understood, then, that the mammas were weighed out of formalin.

The basis of comparison between the meat- and the bread and-milk-fed rats is the percentage weight of the mammary tissue to that of the mother.

By a reference to the tables below it will be seen that there are wide individual variations in this percentage among the bread-and-milk animals.

Table Giving The Lactation History Of Eight Rats Fed On An Exclusive Ox-Flesh Dietary

No. of Days on Meat before

Parturition.

Weight of Animal at Death.

Weight of

Mammae.

Per cent.

Time in

Lactation when Killed.

Number of Young.

Weight of

Young.

Grins.

Grms.

Grms.

30

180

13

7.2

1st day

10

50

9

140

8.5

6

1st day

10

49

27

230

19.5

8.4

1st day

..•

..

25

150

13

8.6

7th day

5

48

12

115

10.5

9.1

20th day

6

120

15

130

12

4.2

21 st day

7

185

2 I

110

10.1

9.1

21 st day

6

110

24

140

11.5

8.2

22nd day

4

82

Average percentage = 8.2.

This is explained by the different times during the course of lactation at which the animals were killed, and also by the varying numbers of young which they suckled. Thus there is a fairly uniform rise in the percentage from the first up to the twenty-first day, after which there is again a fall; and taking animals killed at the same lactation period, the percentage is higher in those which nursed the larger number of young.

We may therefore take it that the mammary gland in the rat is most actively functioning about the twenty-first day of lactation, and alter this, as the young begin to feed themselves, it undergoes atrophy.