1. Of 583 pig foetuses of all ages, 331 were found to be males and 252 females, giving a male percentage of 56·8, which is much higher than the approximate equality between the sexes that seems to exist at birth. This result confirms Jewell, who found a male percentage of 55 among 1000 cow foetuses.
2. These 583 pig foetuses when classified by weight (0–100 gm., 101–300 gm., 301+gm.) gave the following percentages of males for the respective groups, 59·1, 57·0, 53·2. In other words, the percentage of males decreases as gestation proceeds, a result which can only be brought about by a differential mortality of males and females. It would appear that the percentage at conception must be very near 60 per cent. or 150 males per 100 females.
3. The sex-ratios of foetuses from the left and right cornua show no significant difference, 55·0 for the right and 54·7 for the left.
4. The males were found to average about 7 per cent. heavier than the females.
5. There seems to be an inverse correlation between the number of foetuses in a horn and their average weight. Foetuses from the left cornu have a slightly greater weight than those from the right, and this may be connected with the slightly inferior fertility of this horn.
6. The excess of males at conception is discussed in the light of the implications of the chromosome mechanism of sex determination, and reference is made to the greater mortality of the males.