HISTOGENESIS OF THE LIVER 263 
at first, dropping from an average of 12.5 in an embryo of 15 mm. 
to 7 in one of 28 mm. I think that this rapid and early reduction 
is due to the stretching of the tubules caused by the dilatation 
of the sinusoids among them. The tubules are attached to each 
other and to the mesothelial wall of the liver by strands of mesen- 
chyma which may aid in the process by pulling upon the cylinders 
as the mesothelial wall is rapidly stretched in' all directions. 
These mesenchymal strands can be seen in figure 43 of the liver 
of an embryo 20 mm. long. 
After the embryo reaches a length of from 28 to 30 mm. the 
hepatic cylinders again begin to increase in size. They rise 
rapidly in diameter from an average of 35.6 micra in an embryo 
28 mm. long to 46 micra in an embryo of 50 mm. and 54 micra 
in an embryo of 95 mm. This growth is still noticeable in an 
embryo of 220 mm. in which, however, the cells were too col- 
lapsed to admit of accurate measurement. While the diameter 
of the tubules increases, the size of the lumen and the number 
of cells surrounding it at any given plane as steadily decreases. 
Thus the average number of cells seen in cross section of a cylinder 
at 25 mm. is 8, at 50 mm. 4.6, and at 95 mm. 3.7. This indi- 
cates that the increase in size of the cylinder is due to the growth 
of the individual cells forming it and not to their multiplication, 
as is the early increase in size found in embryos from 13 to 16 mm. 
long. 
Almost all this growth is due to the deposition of fat in the 
hepatic cells, the nuclei of which remain stationary or actually 
decrease in size. While the cells begin to increase in size even 
when the cylinders are becoming reduced in caliber, due to factors 
already mentioned, this growth is not sufficient to make up for 
the reduction until the embryo reaches a length of more than 
30 mm. 
The thick sections illustrated in figures 40, 41 and 42 show 
graphically some of the changes just described. Figure 40 is 
a section of a liver in which the process of anastomosis is well 
under way. It shows the large size and irregular caliber of both 
the cylinders and their lumina. Figure 41 is from a specimen 
in which the hepatic sinusoids have nearly reached their highest 
