470 WILSON. 



PLATE XV. 



[Figs. 23, 28, 29, 32 from life; the others from preparations.] 



Fig. 23. Twenty-nine-celled stage, from upper pole. The trochoblasts have 

 divided radially (completing twelve-celled girdle); the four tertiary micromeres 

 formed, secondary micromeres still undivided (3 h. 43 m.). 



Fig. 24. Succeeding stage, from right side. Second group of micromeres in 

 division. 



Fig, 25. Thirty- two- (four-) celled stage, right side-view. Third spiral cleavage of 

 fli, b^, fi, afi in progress (4.T h.). 



Fig. 26. Anterior view of the same embryo. 



Fig. 27. From the upper pole. Third spiral cleavage of a^, b^, c^, d^ (formation 

 of apical rosette) . Transition from thirty-two- to thirty-six- celled stage. 



Fig. 28. Thirty-six-celled stage, from the upper pole (4 h. 7 m,). 



Fig, 29. Thirty-eight-celled stage, rear view. Second division of X. Origin of 

 second somatoblast (4 h. 55 m.). 



Fig. 30, Stage immediately following the last. First stage of transitional period. 

 The two posterior primary micromeres, c^, d'^, preparing for their first bilateral 

 division (5 h. 15 m.). 



Fig. 31. Forty-celled stage. Third (median dorsal) division of X. Bilateral 

 division of c^, d'^ completed (forming c^'*, d^-'^') (6 h. 30 m.). 



Fig. 32. Forty-three- (typically forty-two-) celled stage. Posterior view of a liv- 

 ing specimen followed from a stage precisely like Fig, 29, Third division of X com- 

 pleted, interruption of the girdle by the displacement of d'^-'^. Fission of the second 

 somatoblast to form the primary mesoblasts, M.M. (5 h. 25 m.). 



Fig. 33. Left side-view of peculiar thirty-celled stage in which the apical rosette 

 is not yet present, though the second somatoblast is formed. Second group of micro- 

 meres just dividing. Second division of ^ not yet effected (7 h.). 



Fig. 34. Optical longitudinal section, thirty-eight-celled stage; position of the 

 left stomatoblast («'^-^) indicated by the dotted line. 



