480 TREA D WELL. 



EXPLANATION OF PLATE XXXVIL 



Fig. 13. Thirty-two cells forming forty, from below. Formation of fourth 

 quartette. 



Fig. 14. Forty cells forming fifty-six, from the side. Second division of the 

 trochoblasts, first division of the intermediate cells, and first division of third 

 quartette. 



Fig. 1 5. Fifty-six-cell stage from the side. Completion of the final division 

 of the primary trochoblasts. 



Figs. 16-23. Successive stages in the history of the cross, ap' to dp', primary 

 trochoblasts. ap" to dp", secondary trochoblasts. ar to dr, rosette cells. 



Fig. 24. Second division of second quartette. Formation of X1.2. 



