654 HAROLD HEATH, 



Fig. IS. Completion of divisions shown in Figs. 16 and 17. 

 Division forming the posterior second quartette stomatoblast. 63 cells. 



rig. 19. Same egg seen from the left side. 



Fig. 20. Vegetative pole, showing the position of the macromeres, 

 third quartette stomatoblasts {Sa'-^, 3b^ etc.), and the posterior secondary 

 quartette stomatoblast (2d^-^). Division of 3a^ and 3b ^ etc. 



Fig. 21. Side view of same stage as Fig. 20. First and last 

 division of the accessory trochoblasts la^-^-^ etc. 64 to 72 cells. 



Fig. 22. Division of posterior macromeres to form mesoblast {4d). 



72 to 73 cells. 



Fig. 23. Division of posterior rosette cells, Ic^'^'-^ and Id^^-^. 

 Early cleavage of basal and median cross cells, /a^^-^-^, la^^^-^ etc. 

 Belated division of accessory trochoblasts. Seen from animal pole. 



73 to 85 cells. 



Fig. 24. Same stage as shown in preceding figure , seen from 

 below. Formation of remaining cells of the fourth quartette. 



Plate 33. 

 (Primary trochoblasts yellow ; mesoderm red.) 



Fig. 25. Same stage as preceding figure, seen from the left 

 quadrant. 



Fig. 26. Posterior second quartette (first somatoblast) seen from 

 the side. Cleavage of third quartette cells and of 2d^-^. 85 to 113 cells. 



Fig. 27. Right quadrant of embryo slightly older than Fig 26 

 showing acceleration of third quartette cleavage in the posterior quadrants. 

 Division of tip cell (secondary trochoblast). 



Fig. 28. Same stage as Fig. 27 seen from vegetative pole. Delayed 

 cleavage of the secondary stomatoblast, 2c'^-^. Division of tertiary 

 stomatoblasts, 3 a-^ etc. Bilateral cleavage of mesoblast. 



Fig. 29. Slightly later stage than Fig. 28 showing cleavage of 

 apicals, la^-^-^ etc. and the anterior rosette cells la^^-^ and Ih^-^^. 

 Basal and median cells of the cross represented, the latter, i«^ -2.^.2 

 etc. dividing. 



Fig. 30. Side view showing cleavage of the superior third quar- 

 tette cells, 3a^-l-l., 3a^-^^^ etc. Division of right and left post-trochal 

 cells, 5«^.^, 2a^'^ etc. Cleavage of 5 a ^. ^. ^ etc. 113 to 149 cells. 



Fig. 31. Animal pole of about the stage shown in Fig. 30. Rosette 

 series (Annelid cross) and basal and median cells of the Molluscan 

 cross (these latter dividing). 



Fig. 32. Vegetative pole showing cleavage of mesoblast cells ; the 

 second divison of the posterior tertiary stomatoblasts; and the cleavage 

 of the posterior secondary stomatoblast. 



Fig. 33. Anterior view showing completion of cleavages represented 

 in Fig. 30. 



Fig. 34. Animal pole showing the rosette series ; the characteristic 

 arrangement of the basal cells of the posterior right, and left arms of 

 the cross; two basal cells, 1 a ^-2. 1.1.2.2 and 1 c 1-2.1.1.1.2^ in a state 

 of division. 



