472 



WILSON-. 



PLATE XVI. 



[All of the figures from preparations.] 



Fig. 35. Transition from forty-two- to fifty-eight- celled stage. View from the 

 upper pole, showing first bilateral division of a^, b^, establishment of the cross, trans- 

 formation of the girdle into the prototroch (5 h. 30 m.). 



Fig. 36. Anterior view of the same specimen. 



Fig. 37. Posterior view, about fifty-eight cells. Second bilateral division of 

 c^, d"^ to form the nephroblasts, c^-^, d^-^. Spindle of fourth division (fission) of 

 X(5h. 30 m.). 



Fig. 38. Complete establishment of bilaterality. Immediately following the last. 

 Formation of the nephroblasts and fission of X completed. Delayed division of 

 c^-^ (5 h. 30 m.). 



Fig. 39. Second bilateral division of a^, b^, forming a^-^, d^-^. Division of c^-*, 

 d^'^. Beginning of the enclosure of the nephroblasts (6 h. 30 m.). 



Fig. 40. Third bilateral division of c'^, d'^ to form c'^-^, d^-^; division of a^-*, <5i-* 

 (7h.). 



Fig. 41. Third bilateral division of a^, b'^ to form dS-^, b'^-^; division of d>--^, b^-''. 

 The nephroblasts nearly enclosed. 



Fig. 42. Fourth bilateral division of c^, d^ to form c^''', d'^-'^; fifth division of X 

 (7 h. 30 m.) 



Fig. 43. Posterior view of the same. 



Fig. 44. Right side-view, somewhat from above, after the fourth bilateral division 

 of c\ d\ At the left upper side the nephroblast is shown in optical section, below 

 ^i'<5 (7 h. 30 m.). 



Fig. 45. Right side-view after enclosure of the nephroblast and the beginning of 

 its migration. Fifth bilateral division of c\ d^. The proteloblasts (XX) have 

 divided (eighth division of X) to form the four posterior teloblasts (X, X, X, X) 

 (8h. 30 m.). 



Fig. 46. Rear view of slightly earlier stage (8 h. 30 m.). 



