106 John H. Geroüld, 



near the dorsal edge, adjacent to the cells of the preoral circlet 

 (Fig. 87, 89). This change of position is probably due to the sinking 

 of the cells which lie immediately dorsal to the rosette, in the process 

 of formation of that thickening of the apical plate which is to con- 

 stitute the supraoesophageal ganglion. The cells of the rosette in 

 Fh. goiddii (Fig. 87, 89) are extraordinarily large in comparison with 

 the surrounding cells of the apical plate. This difference in size is 

 less marked in Ph. vulgare, in which the rosette cells are smaller 

 than in Fh. goiddii. The flagella of the rosette (Fig. 49, 50, 52, 59, 

 60, 62, 65) are of remarkable length, being nearly, or quite, as long 

 as the transverse diameter of the trochophore (0,13 mm in Fh. 

 gouldii). 



The cells of the definitive rosette remain undivided, large, and 

 prominent, during the growth of the trochophore. The cells which 

 surround them, however, increase in number by repeated divisions. 

 At the beginning of this period they form scarcely more than a 

 single layer, but at the close (48 hours) they lie in several layers, 

 of which the deeper form the rudiment of the supraoesophageal 

 ganglion (Fig. 86, 87), which, however, is not yet separated from the 

 superficial ectoderm. 



A pair of pigment spots appear, at about the thirty-sixth hour, 

 in the superficial cells on the dorsal side of the apical plate, one 

 on each side of the body (Fig. 46, 49, 57, 60, etc.). They form 

 hollow hemispheres which open laterally towards opposite sides of 

 the body, and are composed of reddish-yellow granules. 



The two peripheral rows of apical-plate cells bear the preoral 

 circlet of cilia, which in Ih. gouldii are large and important organs 

 of locomotion (Fig. 60, 62, 87, 89), whereas in Ph. vulgare, in which 

 the postoral circlet plays the more important rôle, they are minute, 

 and cannot be readily distinguished from the adorai cilia of the 

 prototroch. 



Tlie prototroch cells are of extraordinary size (Fig. 47, 82). In 

 this respect they much resemble those in Amphitrite, as described 

 by Mead (1897). They consist of sixteen "primary" cells, which 

 originate, as I have already shown, from the first quartet of micro- 

 meres, and of three "secondary" cells. They form a complete girdle 

 about the embryo, which, upon the sides of the body, is very broad. 

 In front of the stomodaeum, however, it is reduced in width, and in 

 the mid-dorsal line it is still narrower. There the cells of the right 

 and left sides touch each other at a single point. 



