292 A Study of the Structural Unit of the Liver 
capillaries the syncytium of endothelium les. We have, therefore, but 
three elements within the liver lobule, liver cells, a syncytium of endo- 
thelial cells, and a network of reticulum between them. 
I have been unable to find any of the fibrils of the reticulum of the 
liver in embryo pigs less than 2 em. long. If frozen sections are made 
of fresh livers at this stage it will be found that they are very delicate 
and can be crushed under the coverglass very easily. When such prepara- 
tions are stained by allowing a solution of magenta to run under the 
coyerglass, it is seen that a network of stained fibrils lies between clumps 
of liver cells. In any such sections it is easy to determine that all of the 
fibrils of the young reticulum surround the capillaries and are in intimate 
connection with Kupffer’s endothelial cells. The fibrils, or rather the 
syncytium is delicate, and can be broken easily by slight pressure upon 
the coverglass. Frozen sections are easily broken into granules by shak- 
ing them slightly in water, showing that the reticulum is not strong. 
When digested for a short time, at room temperature the liver cells dis- 
integrate, leaving only the delicate syncytium to which many small gran- 
ules adhere. Pressure upon the cover glass shows that the reticulum is 
very elastic. Acetic acid does not cause it to swell and become trans- 
parent. 
It is not difficult to obtain fresh specimens with all of the capillaries 
surrounded with this delicate reticulum with the endothelial nuclei im- 
bedded in it. The continuity of the endothelial cells with the embry- 
onic reticulum is complete, and thus forces us to the conclusion that the 
fibrils are developed from the endothelial cells in the same manner as I 
have shown that they are developed from the connective-tissue syncytium 
elsewhere. 
This observation, however, is entirely out of harmony with the develop- 
ment of connective tissues in general, for they arise from mesenchyme, 
while the reticulum of the liver arises from the angioblast. However, the 
reticulum fibrils are in no way connected with the liver cells and there is 
no third group of cells in this neighborhood. It is possible that 
these fibrils reach into the lobule from distant interlobular spaces where 
connective tissue cells may be found. 
If the liver of a dog is carefully crushed with the fingers in a stream 
of water all of the lobules are gradually destroyed and the portal and 
hepatic trees may be separated. At the end of the tips of the portal tree 
there are small enlargements which correspond with the portal units; no 
corresponding lobules are found to adhere to the tips of the hepatic veins. 
The stronger tissue—the capsule of Glsson—is thus isolated, while the 
