CLEAVAGE AND DIFFERENTIATION. 



31 



Fig. II. — 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 trochoblasts bilaterally. (Hey- 

 mons' 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 trochoblasts and the tip cells of the 

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

 (Metcalfs Fig. XIV.) — /, Chiton : apical view of the 48-cell stage, showing the cross, the 

 rosette, and the trochoblasts. (Metcalfs Fig. XXIV.) 



