484 TREA D WELL. 



EXPLANATION OF PLATE XXXIX. 



Figs. 37 and 38. Formation of tertiary trochoblasts, ap"\, ap"'z. 



Figs. 39 and 40. Divisions of X-cells. 



Fig. 41. Stage of Fig. 38 from below. Note divisions of 3a2.2, ap'" to d'", 

 tertiary trochoblasts. 



Fig. 42. Completion of division of X-cells, begun in Fig 40. 



Fig. 43. Beginning of closure of the blastopore, and division of X. Blp., 

 blastopore. 



Fig. 44. Division of X2.2-i and beginning of migration of "1" cells from the 

 upper hemisphere. 



Fig. 45. Division of X-cells and further migration of " 1 " cells. 



Fig. 46. Further closure of the blastopore and division of Xi.2.i. 



Fig. 47. Completed closure of the blastopore. Proc, proctodaeum. Stom., 

 stomodaeum. 



Fig. 48. A little later than 47. Complete concrescence of the X-cells on the 

 ventral surface. 



