At the first cleavage of the egg each of these substances is divided into right and left halves (Fig. 29, 5). The second cleavage cuts off two anterior cells containing the gray crescent from two posterior ones containing the yellow crescent (Fig. 29, 6 and Fig. 30, 1). The third cleavage separates the colorless protoplasm in the upper hemisphere from the slate-blue in the lower (Fig. 30, 2). And at every successive cleavage the cytoplasmic substances are segregated and isolated in particular cells,—and in this way the cytoplasm of the different cells comes to be unlike (Figs. 30 and 31). When once partition walls have been
Fig. 30. Cleavage of the Egg of Styela, showing distribution of the yellow protoplasm (stippled) and of the clear and gray protoplasm to the various cells, each of which bears a definite letter and number.
formed between cells they permanently separate the substances in the different cells so that they can no longer commingle.
What is true of Styela in this regard is equally true of many other ascidians, as well as of Amphioxus and of the frog, though the segrega-
