Joseph Marshall Flint | 441 
places as though composed of a singie layer of epithelium. Other intra- 
lobular ducts have a distinct double layer even at this stage of devel- 
opment. © 
Tn a pig two days old there has been scarcely any change in the char- 
acter of the epithelium and the larger ducts, except that goblet cells 
filled with plugs of mucous stained with aniline blue are now occasion- 
ally found between the elements of the inner columnar layer. ‘These 
can be observed in ducts as high as the sublobular order. Ducts of the 
lobular and intralobular type have in many places a double layer of 
epithelium but show as yet no evidences of the fibrillation observed in 
the intralobular ducts of the adult. Intercalary ducts in some places 
appear to be lined by a single layer of flattened epithelium, while in 
others indications of a double layer are found, but sections that pass 
directly through the lumen of the intercalary ducts ordinarily have 
only one row of epithelial cells lining them. In the adult, the height 
of the columnar epithelial cells forming the inner stratum of the larger 
ducts is considerably increased as well as the number of goblet cells 
found between them. In a two-day pig the goblet cells are rounded, 
while in the adult, unless greatly distended, they are elliptical in form. 
The intralobular ducts in the adult have but a single layer of epithe- 
lium, and the cell pole away from the lumen of the duct is dictinctly 
striated. Intercalary ducts which are extremely difficult to find owing 
to the increase in size and the thickness of the alveolar groups, now 
show usually but a single layer of flattened epithelium. In each of the 
lobules there are a few groups of serous alveoli embedded among those 
of the mucous type. These are provided with intercalary ducts not 
dissimilar from those that enter the mucous alveoli. The serous alveoli 
are extremely few in number and are difficult to find in sections of the 
pig’s submaxillary at the time of birth. But they may occasionally be 
noted in pigs 264 centimeters long coming off from intercalary ducts 
adjacent to a group of the regular mucous alveoli. Whether they rep- 
resent a special differentiated form of alveolus concerned with the pro- 
duction of special elements of the submaxillary secretion is not certain, 
but the method of their development is obvious. They represent simply 
an alveolus in which the cells of the inner layer have not been differ- 
entiated into the cells of the mucous type. It does not follow, of course, 
that they would then be analogous to the parietal cells of the mucous 
alveoli, although this hypothesis must be considered. It is interesting 
to note, however, that in pigs of this age one finds these alveoli with a 
double layer of cells. It should be remembered, moreover, that in pigs 
