1904.] NATURAL SCIENCES OF PHILADELPHIA. 719 



and their changed relation to the entoderm and the X group is the 

 result of the shifting of the ectoderm already described. A clear idea 

 of the change of position of the cells in question may be obtained from 

 a glance at text fig. IV, A-D. The corresponding cells in the B quad- 

 rant have not been seen. Doubtless 2b^-^ divides, as do 2a^-^ and 2c^-^, 

 though at a much later period, but the products of such a division are 

 not to be recognized in fig. 53. In the embryo represented in the 

 figure the trochoblasts can plainly be discerned dorsal to 2a^-^-^ and 

 2c^--*; on the ventral side of the embryo they form an irregular cell 

 row. Below this there is a group of small cells which presents to the 

 eye no definite arrangement. These represent the descendants of 

 2b^--; 3a and 3b. Posterior to the blastopore are those cells which in 

 annelids form the ventral plate. These cells barely cover the posterior 

 face of the primary mesoblasts. Just posterior to 2a^-^-^ and 2c^-^-^ are 

 one or two small cells on each side whose lineage has not been deter- 

 mined, but which are probably descendants of 3d and 3c. Turning to 

 fig. 55, the relations of the cells surrounding the blastopore are much 

 less clear. On each side of the anterior portion of the blastopore are 

 two cells slightly larger than those surrounding them. These are 

 probably the descendants of 2^^-''-^, 2a2-2-2, 2c2-2-i and 2c2-2-2. The 

 blastopore has meanwhile narrowed to an irregular cleft, the cells 

 anterior to it forming a seam, while the cells of the ventral plate have 

 only advanced sufficiently to cover in one of the mesoblasts and part 

 of the other. All of the cells which constitute the rim of the blastopore 

 are small and extremely thin and transparent, making their outlines 

 very difficult to discern, and the exact lineage of any of these impossible 

 to determine. From a comparison of figs. 53 and 55 it is seen that the 

 cells of the ectoderm have crowded toward the vegetal pole from all 

 directions, but especially from the sides. It is also evident that the 

 anterior portion of the margin of the blastopore, between 2a^-^-^ and 

 2c^-^-\ is composed of the descendants of 2b^-^, 3b and 3c; that the 

 lateral margin is at least partly composed of the descendants of 2a and 

 2c, unless the small cells which form the blastopore rim in fig. 55 have 

 all slipped in around 2a--^-^, 2a---^, 2c^-^-^ and 2c2-^ -^ It seems probable 

 that this is not the case, but that the small cells comprising the lateral 

 margin of the blastopore are derived from the second quartette. It is, 

 however, fairly certain that some small cells have pushed in between 

 the descendants of 2a and 2c and the cells of the X group. These must 

 be the descendants of 3c on one side and 3d on the other. 



In Nereis, Wilson (1892) described these cells 2a^, 2c? and 2b^ as 

 "stomatoblasts," since they converge to form an arc of cells as the 



