The Development of Ischnochiton. 653 



The following reference letters have been used : 



Ä Tegmental sense organ. Mv Mucous cells. 



Bip Blastopore. Oc Eye. 



Ca Lime-secreting cells. Sq Second quartette. 



Cg Cerebral ganglia. Sh Shell. 



Fg Foot gland. St Stomodaeum. 

 The primary trochoblasts in Plates 2 and 3 have been colored 

 yellow, and the mesoblast red. 



Plate 31. 



Fig. 1. Egg of Ischnochiton magdalenensis ^ side view showing 

 first cleavage spindle and a portion of the chorion. 



Fig. 2. Late stage in the second cleavage seen from above. 



Fig. 3. Completion of second cleavage. Polar furrows represented. 

 4 cells. 



Fig. 4. Third cleavage, showing leiotropic position of the spindles. 



Fig. 5. Completion of the first quartette. Segmentation cavity. 

 8 cells. 



Fig. 6. Fourth cleavage. Formation of primary trochoblasts and 

 of second quartette. Animal pole. 



Fig. 7. Later stage. Slightly accelerated division of the first 

 quartette. 16 cells. 



Fig. 8. Completion of primary trochoblasts and of second quartette. 

 Second division of first quartette and formation of a third and last 

 quartette of ectomeres. 



Fig. 9. Same egg as Fig. 8 seen from above. 



Fig. 10. Same stage as Fig. 8 seen from below. 



Fig. 11. Position of macromeres seen after the formation of the 

 third quartette. Vegetative pole. 24 cells. 



Fig. 12. Later stage showing shifting and slight invagination of 

 the macromeres. 32 cells. 



Plate 32. 

 (Primary trochoblasts yellow; mesoderm red.) 



Fig. 13. Side view showing division of primary trochoblasts and 

 of the second quartette. 



Fig. 14. Completion of cleavage shown in the preceding figure. 

 Division of la^'-^ etc. forming the accessory primary trochoblasts, first 

 stages in the division of the apicals, la^- ^ etc. 32 cells. 



Fig, 15. Formation of the accessory primary trochoblasts. 



Fig. 16. Division of la^-'^-^ etc. to form basal and median cells 

 of the cross. Cleavage of each primary trochoblast la'-^- ^^ la'-^ '-^^ Ih'-^ J^^ 

 Ib^-^ etc. Division oî 2a^, 2h^ etc. to form the tip cell of the cross 

 and the right post-trochal cell. First stages in the division of 2«^, 

 2h'^ etc. First cleavage of the third quartette forming stomatoblasts. 

 38 to 62 cells. 



Fig. 17. Slightly later stage than Fig. 16 seen from animal pole. 



