ON THE EARLY DEVELOPMENT—BIPOLAR DIFFERENTIATION AND CLEAVAGE—OF THE ZEBRA FISH, BRACHYDANIO RERIO

¹ From the Biological Department, Brown University

A detailed description of the early development of the egg of B. rerio is presented. The observations are made chiefly on the living egg. The measurements are made on a moving picture, taken by time accelerator. This proved to be a valuable help in discovering facts which could not be detected by simple observations. After fertilization the egg as a whole takes up water, but the cell itself shrinks. Later the cell itself begins to swell and to take up water.

The bipolar differentiation is first indicated by the position of the nucleus and a small amount of protoplasm around it. A conspicuous separation of protoplasm from yolk begins when the cell first takes up water and ceases during a complete resting phase before the first mitosis. With the beginning of the first mitosis a separation process of different nature takes place. This consists in a streaming of protoplasm through the yolk towards the blastodisc. In addition, a counterstream towards the vegetal pole is clearly visible in the film. The changes in the physical state of the cells connected with the mitoses are immediately correlated with the rhythmic separation process.

It is possible to orient the egg at the time of fertilization not only with respect to the polar axis but also with respect to the first cleavage plane.