266 
in size is due to enlargement ot the cell by the formation or cavities 
in the yolk. 
Fig. 1. Fig. 2. 
Fig. 4. Fig. 5. Fig. 6. 
Only such figures have been inserted as seemed necessary to explain the text. 
Figures of all intermediate stages will accompany the completed account. All figures are 
from camera tracings, drawn to the same scale. Differences in size are due to variations. 
in size of eggs. The vitelline membrane has not been shown. The ectomeres are desig- 
