1904.] NATURAL SCIENCES OF PHILADELPHIA. 363 



Synchronously with these changes the blastopore continues to de- 

 crease in size, being narrowed by overgrowth of cells in that neighbor- 

 hood. It will be seen by the examination of fig. 78 that the large 

 cells of the third quartet in the anterior quadrants (3a"^, "^, ^-\ ^^^ and 

 3b"^, "-, ^-^, ^^^) are all encroacliing farther upon the smaller cells of 

 the same series, which have been crowded beneath them at the edge 

 of the blastopore. Posteriorly, derivatives of the third quartet have 

 completely surrounded the blastopore by the division and migration 

 backward of the small cells 3c^ and 3d-, while more laterally the re- 

 maining small cells of this quartet and their neighboring larger cells 

 are crowding aroimd the depression. The second quartet cells, 2a^^ 

 and 2c22, or their derivatives, yet lie in the lateral corners; but as 

 closure of the blastopore proceeds they are crowded from this position 

 by encroachment of the third quartet both from before and behind, 

 which finally (fig. 79) join each other on the sides. In the anterior 

 median plane, however, a cleft yet remains between the large third 

 quartet cells, and after the inner of these large cells have divided, as 

 shown in fig. 79, cells of the second quartet, represented by the deriva- 

 tives of 2b^^, still occupy the space between them and there bound the 

 blastopore. Throughout this process the greatest extension of the third 

 quartet is manifest in the area covered by the posterior third quartet 

 groups, and this is doubtless connected \vdth the disappearance from 

 the ectoderm in the anterior groups of the secondary mesoblast. The 

 blastopore closes from behind forward, to which process the larger 

 number of third quartet cells in the ectoderm of the posterior region 

 conduces. 



The posterior surface of the gastrula is now covered by large cells 

 of the third quartet, and in the median region by second quartet 

 elements. On the right posterior surface (left when seen from ventral 

 surface, fig. 79) may be seen one very large cell, Ex. (3c""), which will 

 later become the principal excretory cell of the larva. The region 

 anterior to the blastopore has been formed from the second quartet 

 cells of B quadrant which have been pushed backward by posterior 

 and apical growth, space being left for them through the shiftmg of 

 the large cells of the third quartet already described. The second 

 quartet cells of B quadrant have shown comparatively little division 

 or growth, and thus appear to occupy a relative!}^ smaller space than 

 previously. 



The blastopore of Crepidula (Conklin) is surrounded by second and 

 third quartet cells, all quadrants contributing. The same is true for 

 Ischnochiton (Heath). In Trochus (Robert) third quartet cells are 



