486 TREADWELL. 



EXPLANATION OF PLATE XL.^ 



Fig. 49. Beginning of invagination of 3a2.2 {l.m.m.). 



Figs. 50 and 51. Division of 461 and 4d2 and of larval mesoblast. l.m.m., 

 median larval mesoblast ; l.m.r., right larval mesoblast ; l.m.L, left larval meso- 

 blast. 



Fig. 52. Second division of larval mesoblast. 



Fig. 53. Final separation of definitive mesoblast and migration of larval 

 mesoblast. Note migration of cells from upper hemisphere. 



Fig. 54. Completion of divisions of Fig. 53. 



Fig. 55. Next division of l.m.m. l.m.r. and l.m.m. have divided. 



Fig. 56. Stage of Fig. 54 from the side. 



Fig. 57. First division of definitive mesoblast. Note migration and large 

 size of "1 " cells. 



Fig. 58. Completion of division of h and division of X3.2. 



Fig. 59. Stage of forty hours from below, ^f., mesoblast bands. 



Fig. 60. Optical sagittal section of stage of forty hours, en., entoderm ; ec, 

 ectoderm ; ad., adoral zone ; ap., apical cilia ; pr.ap., preapical cilia ; /;-., prototroch. 



1 In Figs. 49 and 50, 2bj.j.,. should read sbj.^.j.j. 



