fas) 
| 
_— 
A Study of the Structural Unit of the Liver 
His. At this time the umbilical veins are broken in their course, having 
already passed the first stage of their growth, and from now on are des- 
tined to pass through the liver rather than past it (Fig. 8). It is inter- 
esting to note that the primary sinusoidal liver—that portion which arises 
with the omphalo-mesenteric vein—is formed while the umbilical vein 
empties directly into the ductus Cuvieri. The process is at its height in 
an embryo of about the same age (No. 76), as shown in Figs. 9 and 10. 
The growth of the liver around and through the omphalo-mesenteric 
veins is accompanied by the growth of capillaries from this vein into the 
new anlage. Hand in hand with this process the circulation through the 
Fic. 6. Section through the third occipital myotome of a human embryo 
2.1 mm. long (No. 12 of my collection). X50. O, third occipital myotome; 
coe, coelom; v, vein; st, septum transversum; /7, liver; ph, pharynx; wv, um- 
bilical vesicle. 
omphalo-mesenteric veins is further reduced by the growth and enlarge- 
ment of the umbilical veins. A double force is at work: blood is diverted 
by the umbilical vein which is gradually assuming greater importance 
and by the capillaries which supply the embryonic liver. According to 
Thoma’s first law, the diminished rapidity of the circulation is followed 
by a reduction of the lumen and in order to accomplish this reduction in 
the present case, the liver sprouts first grow into the vein instead of 
around it. The operation of Thoma’s law in this case is so extensive that 
it reduces a main trunk to capillaries which forms a condition recognized 
by Minot as a sinusoidal circulation. 
At the time the liver circulation is entirely sinusoidal, 7. e., about the 
