488 Edwin G. Kirk. 
notin height, so that they become thick, short and cylindric, or 
pyramidal, with broad, basal ends. The nuclei are ovoid, or often 
almost spherical and moderately chromatic. In glands of the 
stage indicated in figs. 6 and 11, the gland cells average 16u in 
length, 8u breadth; the ridge cells, 32u height, 3.24 breadth at mid- 
dle, 6.4 breadth at the broadest part ordistalend. Up tothestage 
of fig. 3 the only distinction between the epithelial cells of the 
pits and ridges, are those just described, of size and of contour of 
cell and nucleus. The cytoplasm of all these cells is quite refractile 
owing to the presence of very fine, slightly acidophile, granules. 
Up to this stage, then, we have found two factors coéperating 
in the deepening of the primitive gland tubules. (1) Upgrowth of 
the mesodermic gland processes, (2) increased height of the ridge 
epithelial cells, which are now over twice as high as the gland 
cells. 
But as the mesoblastic buds reinforce the epithelial gland 
processes mitoses become relatively fewer in the ridge cells, and 
appear inthe pit cells, and it soon becomes apparent that the gland 
tubules elongate by interstitial growth, i.e., by mitoses of the gland 
cells. ‘Thus the epithelial part keeps pace with the growth of the 
lamina propia.. 
The chronological priority, however slight, of the epithelial 
ridges to the mesoblastic cores, seems, to indicate that, from the 
first, certain epithelial cells possess an inherently different po- 
tentiality from others. For the primary ridges and depressions 
are so minute that it is inconceivable that differences in the 
distribution of vascular supply in the underlying mesoblast 
could account for this appearance. But, waiving this, at present, 
fruitless discussion of the predetermination of the cells, it is 
obvious that the grosser differences in the external form of the 
ridge and pit-cells are largely a function (in the mathematical 
sense) of the formation of ridges and pits, i. e.,—the inverted 
pyriform contour of the ridge cells is largely the mechanical re- 
sultant of the compressive forces exerted at their base end and of 
tensile force exerted at their distal end; and the short, pyramidal 
form of the gland cells, of compressive forces exerted at their 
distal end, and of tensile force applied at the base. And these 
