Changes of the external and internal pigment pattern upon fertilization in the egg of Xenopus laevis

External and internal pigment shifts in Xenopus laevis eggs were studied between fertilization and first cleavage. Externally visible, constant features are: (1) the 'activation contraction', a pigment shift towards the animal side taking place between 5 and 15 min post fertilization (p.f.) and (2) the concentration of the pigment around the sperm entrance point leading to the formation of the grey crescent at the opposite side of the egg. Hence, in Xenopus the grey crescent is not formed by rotation of the pigmented cortical layer with respect to the internal egg mass. Histological examination reveals that during the activation contraction the pigment is mainly concentrated in the cortical cytoplasm. Except in the region around the sperm entrance point, from 15 min p.f. onwards, the pigment progressively disperses through the subcortical layer and part of it even moves more deeply into the egg. After fusion of the pronuclei (45--60 min p.f.) the pigment in the subcortical layer forms aggregates. During the pigment shift the yolk-free cytoplasm is displaced dorsally and is ultimately found opposite the sperm entrance point. Thin fibrillar structures in the yolk-free cytoplasm progressively orient themselves parallel to the dorso-ventral plane, and from 40 min p.f. onwards towards the pronuclei. These observations are discussed in connexion with cinematographic observations by Hara, Tydeman & Hengst (1977).