The animal pole plasm in the eggs of Limnaea stagnalis becomes visible about 1 hr. before the first cleavage. At this time the male pronucleus is on its way toward the animal pole, while the female pronucleus is being formed by the fusion of karyomeres resulting from the swelling of the egg chromosomes after the completion of the second maturation division.
The animal pole plasm forms a layer of protoplasm immediately beneath the egg cortex in the animal hemisphere, staining dark violet blue with iron hematoxylin. Contrary to the rest of the cytoplasm, it contains no vacuoles. It is, however, very rich in mitochondria (Raven, 1945) (Plate 1, A).
In eggs centrifuged before the first maturation division, an animal pole plasm may be formed at the normal time and in its normal location, irrespective of the stratification of substances brought about by centrifuging.