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 the 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 stomodseum, 

 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 row^s 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 distinct 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 wliich 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 lar\-a 

 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 



