THE DEVELOPMENT OF A MOLLUSK 53 



13. Sixth cleavage complete. Division of second quartet com- 

 plete. Quadrangular plate of ectomeres with angles of plate in 

 furrows between macromeres. Twenty ectomeres (4 apical, 4 

 turret and 12 belt cells) and 4 macromeres. Jour. Morph., Vol. XIII, 

 figs. 19, 20. 



14. Formation of first member of fourth quartet, the mesento- 

 blast, from the left posterior macromere; formation of basal cells of 

 cross by the second division of first quartet. Jour. Morph., Vol. 

 XIII, figs. 22, 23. 



15. Division of the mesentoblast completed, dexiotropic. Second 

 and third quartets. Turret cells formed. Forty-two cells: 4 apicals 

 8 cross, 4 turret, 20 belt cells, 2 mesentoblasts, 4 macromeres. Jour. 

 Morph., Vol. XIII, fig. 29. 



16. Fourth quartet completed by laeotropic cleavage of macro- 

 meres, A, B and C. The two mesentoblasts of the preceding stage 

 have divided, forming the two enteroblasts and two primary meso- 

 blasts which lie immediately above the latter, but concealed by the 

 plate of ectoblasts. Jour. Morph., Vol. XIII, fig. 31. 



17. Further division of ectoblasts. Expansion of arms a, b and 

 c of ectoblastic cross into a cell plate. Anterior shifting of apical 

 cells. Posterior turret cells undivided. Formation of quadrangular 

 blastopores, the enteroblasts in posterior angle. Jour. Morph., Vol. 

 XIII, figs. 51, 52. 



18. Later stage. Apex on ventral side, slightly to the right. 

 Cells of ectoblastic cross, first quartet, cover the whole anterior end 

 of embryo. Large cells of posterior arm, dorsal. The closing of 

 the blastopore and a depression about it indicating the formation of 

 the stomodasum. The superior rows of ectoblast cells of second 

 quartet, directly above the blastopore, form the first and second velar 

 rows. The"shell gland is forming at the postero-dorsal and somewhat 

 to the left. Jour. Morph., Vol. XIII, figs. 65, 74, 75. 



19. Older embryo, showing apical, posterior and pedal cell plates. 

 On either side to the anterior and posterior of the dorsal cell plate, 

 the velar rows are branching. I\Iouth and the external kidneys are 

 formed, the shell gland expanding. Jour. Morph., Vol. XIII, figs. 

 76 to 79. 



20. Older stage-formation of velum and foot. The shell gland 

 greatly [expanded and forming the shell of the veliger. Jour. Morph., 

 Vol. XIII,' figs. 80-82. 



21. The fully formed veliger. 



22. Section of the preceding. 



[29] 



