No. I.] THE EMBRYOLOGY OF CREPIDULA. 



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a 



Diagram 12. — The cross in Neritina, Umbrella, and Chiton. — a, Neritina : three cells in each 

 arm except the posterior ; the granular tip cells of the transverse arms are the " Urvelarzellen." 

 (Blochmann's Fig. 53.) — b, Neritina; four cells in the posterior arm, three in each of the 

 others. The probable origin of the outer belt cells is indicated by arrows, and the designation 

 of the cells in this and in the preceding figure are given as in Crepidula. (Blochmann's Fig. 56.) 

 — c, Umbrella : the arms of the cross are stippled ; Heymons' so-called " cross " is shown in 

 heavy outline. (Heymons' Fig. 14.) — d. Umbrella: stippling and outlines as in c. The 

 basal cells in the arms of the cross have divided laeotropically, the turret cells bilaterally. 

 (Heymons' Fig. 20.) — e, Chiton : lateral view of the 32-cell stage. The small cells around 

 the equator of the egg correspond in origin and position to the turret cells and the tip cells of 

 the gasteropod ; they should form the prototroch if they have the same destiny in the two cases. 

 (Metcalf's Fig. XIV.) — f, Chiton ; apical view of the 48-cell stage, showing the cross, the 

 rosette, and the turret cells. (Metcalf's Fig. XXIV.) 



