414 TREADWELL. [Vol. XVII. 



very considerably. It immediately adjoins the cell X3.2, and the 

 two form a landmark which is unmistakable. Figs. 58 and 59 

 (PI. XL) show that the cells which have migrated through the 

 prototroch eventually comprise a large portion of the sub- 

 umbrella, fully three-fourths of it being formed from them. 

 The cell li divides transversely. (See the products of its divi- 

 sion, li.i andli.2, inPl. XL, Figs. 58 and 59.) Beyond the latter 

 stage I have not followed it. All of these cells become excess- 

 ively thin and their outlines difficult to follow, so that in the 

 later figures I have indicated only the nuclei. The entoderm 

 cells lie close under the ectoderm, the two being dorsally in 

 actual contact, and it frequently becomes impossible, except in 

 optical section, to determine whether a given nucleus belongs to 

 ectoderm or entoderm. In PI. XL, Figs. 58 and 59, I have 

 intended to represent only ectoderm nuclei. 



A similar migration has been recorded in Amphitrite by 

 Mead (No. 22), but there the number of migrating cells is 

 much less than in Podarke, and they play a much less impor- 

 tant part in the formation of the subumbrella. 



The Cross. — In the 64-cell stage the cross is composed 

 of four equal cells, radially arranged. At the next division 

 bilateral symmetry appears. It is interesting to note that this 

 is accomplished by a dexiotropic cleavage ; the direction which 

 it ought properly to assume according to the law of alternating 

 cleavages. Fig. 16 (PI. XXXVII) shows the beginning and 

 Fig. 17 (PI. XXXVII) the completion of this division. In the 

 latter is seen, also, the beginning of the next divisions of the 

 cross cells in two quadrants. In the latter figure, also, is shown 

 the division of the upper (sinistral) intermediate cell in each 

 quadrant. The result of the first bilateral division of the cross 

 cells is shown in PI. XXXVII, Fig. 17. Two of the cells 

 have divided unequally, two very nearly equally, so that a ver- 

 tical plane can be drawn which would cut the embryo into 

 bilaterally symmetrical parts. This plane of symmetry coin- 

 cides with the plane marked out by the small cell X1.2, pre- 

 viously described at the 64-cell stage. The median plane 

 of the embryo is now fully indicated, and orientation is com- 

 plete. The constant position of the polar furrow up to 



