388 PROCEEDINGS OF THE ACADEMY OF [April, 
enlarged and prominent. This is the region of posterior growth, and 
from this area arise both the shell-gland and the foot. 
Taking up first the history of the former of these two organs, it will 
be found that in a stage represented by figs. 95 and 98 thé whole 
area between the blastopore and the end of the posterior arm of the 
cross shows karyokinetic activity, but particularly in the region marked 
Sh.G. the cells have increased considerably in size. As growth con- 
tinues these cells upon the upper and posterior surface of the gastrula 
protrude above the level of the ectoderm, the area which they cover 
having the appearance of a rough cobble-stone pavement; but some- 
what later they settle down and form a smooth surface. The center of 
this area, which now lies just opposite the region of the stomodeum, 
begins to invaginate, pushing the enteron before it and reducing its 
cavity, so that there results a deep pit which, growing in size below, con- 
stricts above, and around which are several rows of large granular cells 
(fig. 102). Such a condition lasts but a short time, for soon the invagi- 
nated area opens outward, the whole forming a large thick-walled 
cap upon the posterior end of the veliger, constricted around its edge 
and merging abruptly with the thin-walled ectoderm anterior to it (fig. 
104). As growth proceeds the shell-gland spreads and becomes much 
thinner, while the larval shell appears as a secretion of the large cells 
which compose it. As the shell continues to extend over the veliger 
its outer edge is marked by several rows of large cells, which by their 
secretive activity lay down the substance which forms the shell (figs. 
105, 106, 107). Almost from its origin as a distinet structure the shell- 
gland is slightly displaced to the left side of the body, and as it increases 
in extent this lack of bilateral symmetry becomes more marked (fig. 
107). 
The ventral prominence which develops into the foot arises some- 
what later than the shell-gland, and the cells which go into it come from 
the second quartet of D quadrant and the third quartet of C and D 
quadrants. The large ectodermal excretory cell, which in the larva 
lies just behind the foot, serves as a guide to show that much of the 
foot, like this cell, arises from C quadrant of the third quartet; and 
though no such landmark is present on the other side, the early history 
of the two quadrants are so similar that we may reasonably suppose a 
like origin from the third quartet for the left side of the foot. Lillie 
has derived the foot of Unio from cells of the second quartet, and Conk- 
lin appears to have done the same for Crepidula. Holmes states for 
Planorbis that as the cells immediately behind the blastopore are of 
third quartet origin, probably the ‘median portion of the anterior end 
