No. 3-] PODARKE OBSCURA VERRILL. 413 



be distinguished I have designated each by/, with the appro- 

 priate letter and number (PL XXXVIII, Fig. 25-Pl. XL, Fig. 

 58). In later stages, where the primary and secondary, and 

 still later, where also the tertiary prototroch cells are not to 

 be distinguished from one another, I have indicated the proto- 

 troch merely by /, with the appropriate letter to show to 

 which quadrant each portion belongs. 



The Intermediate Cells. — The last division of these cells was 

 leiotropic. The next is dexiotropic and leads to the forma- 

 tion of four cells in each quadrant (PI. XXXVII, Figs. 17 

 and 18). In three of the quadrants (a, b, and c) the lower 

 product of the division of the dextral cell iCi.,-1.2.2, etc., com- 

 pletes the pri7nary prototroch (PI. XXXIX, Figs. 37 and 38, ^/", 

 etc.). They pass into the space between the sets of primary 

 trochoblasts, become ciliated, and function as prototroch cells. 

 I propose to call them the secondary trochoblasts. The other 

 products of this division fill the space between the arms of the 

 cross and become a part of the umbrellar ectoderm. For some 

 few divisions I have been able to follow their history, but after 

 the cross cells begin to divide into small cells the two sets 

 intermingle so that it is not possible to distinguish them. 



In the fourth quadrant the history of the intermediate cells 

 is very different from that just described. Their first division 

 into four cells is the same, but a little later the lower product 

 of the division of the dextral cell, i.e., the one corresponding to 

 the cell which in other quadrants becomes a part of the pro- 

 totroch, begins to migrate through the opening in the proto- 

 troch, and thus forms a portion of the lower hemisphere. In 

 this migration it is accompanied by part, at least, of its sister- 

 cell, but I think by none of the descendants of the other inter- 

 mediate cells. These migrating cells push through the break 

 in the prototroch, dividing as they go (PI. XXXIX, Figs. 42, 44, 

 and 45 ; PI. XL, 53, 57, and 58, h, I2, etc., and the cells just dorsal 

 to them). Since the opening persists until late in the develop- 

 ment, the process of migration lasts for a considerable time, or 

 until the trochophore is fully formed. In PI. XL, Figs. 53, 57, 

 and 58, it will be seen that these cells flatten out as they pass 

 downward, one of them, h (PI. XL, Figs. 53 and 57), elongating 



