THE DEVELOPMENTAL HISTORY OF AMAROECIUM CONSTELLATUM. 1. EARLY EMBRYONIC DEVELOPMENT

¹ Marine Biological Laboratory, Woods Hole, and the Biology Department, Seton Hill College, Greensburg, Pennsylvania

1. The egg of Amaroecium contains more yolk than that of any of the other Ascidians whose embryology has been studied.

2. In the two cell stage the right blastomere is always smaller than the left, establishing an inequality in size that persists through subsequent divisions.

3. In the fifth division the cytoplasmic substances are distributed in this fashion: ten ectodermal cells at the animal pole, four endodermal macromeres at the vegetative pole, a crescent of two chorda-neural cells between them on the anterior side, a crescent of six mesodermal cells on the posterior side, two endodermal-mesodermal cells on each side.

4. Decreased size of cells and increased activity in the mesodermal cells on the right side produce asymmetry in the blastoporal lip.

5. Gastrulation is accomplished by the combined processes of overgrowth and invagination.

6. The blastopore closes from right to left, producing a curve in the neural plate through an angle of 90° in the region of the potential neural tube.

7. When the blastopore closes the potential muscle cells lie above and below the notochord, interrupted on the left by the neural tube, on the right by the endodermal rod.