6 Reticulated Basement Membranes in the Submaxillary Gland 
tips of the growing tree is having a certain influence on the course and 
direction of the adjacent fibrils, and they, in turn, have influenced the 
position of the cells that lie either at their nodes or along their course. 
Relative to the entire syncytial mass, the growing tree forms a com- 
paratively small proportion of the embryonic gland, so that either the 
force exerted by the growing columns is lost in the plastic mass or the 
exoplasm has not developed a sufficient strength to transmit it any dis- 
tance from the foci where it is exerted. In the syncytium are numer- 
ous spaces which are quite clear and are probably filled with fluid. The 
basement membrane is more distinct at this period than in a pig 3 cm. 
in length, but the presence of so many syncytial cells immediately about 
the growing columns is no longer so frequently observed. Now an 
occasional nucleus with its endoplasm rests directly on the membrane 
and it is noteworthy that there is a marked diminution in the quantity 
of endoplasm about the nuclei. From this structure extending into the 
adjacent syncytium and forming with it a direct fibrillar continuum are 
the exoplasmic fibrils which are so numerous 
in this region, that, as the columns grow out 
into the general syncytium, it is impossible 
that fibrils should not be laid down by thou- 
sands.on the surface of the advancing columns. 
In a pig 8 em. in length the cell columns 
show at their apices the beginning of a differ- 
entiation destined to mark the future alveoli. 
In the terminal buds a lumen has appeared 
and the cells are divided into two general, but 
more or less indefinite layers. Within the 
cells of the inner layer globules of mucus 
begin to appear making it a very simple mat- 
ter to distinguish the alveoli from other por- 
tions of the gland by the presence of these 
characteristic mucous cells. The basement 
membrane is now somewhat thicker and more 
definite. Thousands of distinct fibrils run 
Fig. 3. Alveolus of grow- 
ing submaxillary of pig 8 em. 
long showing formation of 
mucous cells from the inner 
cellular layer of the alveolus. 
Basement membrane and syn- 
cytium of the deposition zone 
show the enormous numbers 
of fibrils that are being laid 
on the surface of the alveolus 
asit grows out into the gland- 
ular framework. Fixed in 
Zenker’s fluid. Stained by 
Mallory’s method. Magnified 
900 diameters. 
from the adjacent syncytium to lose them- 
selves upon the growing membrane. In this, 
as in the earlier embryos, the deposition zone 
is quite as clearly marked out and consists 
now entirely of distinct, sharply-defined, exo- 
plasmic fibrils, containing a few nuclei lying immediately adjacent to 
the alveolus and surrounded by a little granular endoplasm. 
The nu- 
