Retarded development of XO conceptuses during early pregnancy in the mouse

Summary. The growth and development of XO and XX mice were compared from 7¼ to 18½ days of gestation. The 7¼-day XO egg cylinders were retarded in development (and consequently small) when compared with XX egg cylinders, and this lag in development remained until 10½ days. By 12¼ days there was a considerable degree of 'catch-up', but this was not fully maintained. A subgroup of very severely retarded XO fetuses were preferentially located near the cervix. The placentas of XO fetuses were of normal size until 18½ days when they were significantly larger than those of XX controls.