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, 4%, "1, and 
db, M2, wt 122) are all encroaching 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 around the depression. ‘The second quartet cells, 2a” 
and 2c”, 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 oceupy 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 with 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 shifting 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 relatively 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 
