The Fate of Polar Bodies in the Egg of Urechis unicinctus

1. It is aimed in this paper to re-examine the fact about the polar bodies in the egg of Urechis that they are said to be enclosed within the blastocoele during early stages of development, and to find out futher, if possible, its probable causes. 2. The enclosure of the polar bodies within the blastocoele occurs at the stage of 16 cells. In most cases, only one polar body is enclosed, but, in quite rare cases, two are enclosed. Polar body or bodies thus enclosed can remain as such even until the stage of young trochophore. Since their detaching or dislocation occurs because of their connection with the egg surface by a stalk, the enclosure of polar bodies within the blastocoele indicates that the original egg surface does not necessary form the outer surface of the embryo of later stages, but, some part of it is involved in the formation of the inner surface of the wall of the blastocoele. 3. Occurrences of the regional shrinkage or the expansion on the surface of the dividing Urechis egg are conceivable from the observations on the behaviors of plasmic strands as well as polar bodies. Curves obtained by the measu rements of distance changes between two polar bodies show that the furrow region of the dividing Urechis egg has far higher expansive capacity when compared with that of the seaurchin. 4. The higher capacity of expansion in the furrow region and the less extent of flattening on the side of the blastomeres of the Urechis egg are suggested as probable causes for the enclosure of polar bodies within the blastocoele