4i8 TREADWELL. [Vol. XVII. 



cross cells (see above, p. 417), and he figures them in Figs. 93 

 and 105. In Podarke, as already stated, the polar globules have 

 a similar fate, but long before the stage corresponding to 

 Mead's Fig. 105 they have completely disappeared. On the 

 other hand, these small cells derived from the ectoderm migrate 

 inward and occupy a position very similar to that of the " polar 

 globules " of the latter figure. This leads to the suggestion 

 that possibly both processes may occur in Lepidonotus also, 

 and that the small cell figured in Fig. 105 is derived from the 

 ectoderm. 



In the dorsal arm of the cross, the cells ici.i.2.1.2 and 

 idi.i.2.i.i next divide (PI. XXXVII, Fig. 20), and this is 

 followed by the division of ici.i.2.1.1 and idi.i.2.1.2. (See 

 PI. XXXVII, Figs. 2 1 and 22, and table, p. 438.) The large termi- 

 nal cells of the dorsal arms divide considerably later, the divi- 

 sions being nearly equal. Figs. 22 and 23 (PI. XXXVII) show 

 the division in C quadrant. (See also PL XXXIX, Fig. 42, 

 idi.i.2.2.2.2.) Later than this I have not attempted to follow 

 the cells in detail. They all enter into the formation of the 

 ectoderm on the dorsal side of the umbrella, in which I have 

 been able to make out no specialized organs. Ventrally there 

 are two "eye-spots" and the large "problematic" organs 

 which develop from either the ventral cross cells or from the 

 intermediate cells. It is impossible to tell from which. 



TJie Rosette Cells. — At the 64-cell stage these are four in 

 number, lying between the cross cells and considerably elon- 

 gated inward. (See PI. XXXVII, Figs. 16, 17, and 18, 

 and Text-Figs. 3 and 4.) Soon after the stage represented 

 in PI. XXXVII, Fig. 17, one of them, invariably the one of 

 the A quadrant, comes to the surface and divides equally 

 (PI. XXXVII, Fig. 18, ar). Next, the one in the D quadrant 

 divides (PI. XXXVII, Fig. 20, dr). Later, the two remaining 

 cells divide, and the resulting plate of eight cells is shown 

 with stippled nuclei in PI. XXXVII, Figs. 22 and 23. These 

 cells differ so little in size from those around them that the 

 two are hard to distinguish, but I think the arrangement 

 given in PI. XXXVII, Fig. 23, is correct. The cells elongate 

 slightly inward (Text-Figs. 5 and 6, ar). From them arises the 



