ON DETERMINATIVE CLEAVAGE AND YOLK FORMATION IN THE HARPACTID COPEPOD TISBE FURCATA (BAIRD)

¹ From the Zoölogical Laboratory, State University of Iowa, and the Marine Biological Laboratory, Woods Hole, Mass.

In the development of Tisbe furcata, a harpactid copepod of the Woods Hole region, there occur two periods of yolk formation. Egg yolk is accumulated in the usual way during ovogenesis. After gastrulation and during the differentiation of the nauplius, yolk is deposited again in the endoderm, which partly assumes the character of a yolk sac.

As in other non-parasitic copepods, endoderm and germ cells originate from a common "stem cell" of the 32-blastomere stage. Tisbe furcata is peculiar in that an equational division precedes the one that separates the endodermal from the germinal line. In the stem cells the segregational division is prepared by a polar differentiation of the cytoplasm which is most clearly indicated by the asymmetrical distribution of the mitochondria.