Franklin P. ‘Mall - 251 
animals.” It seems to me that there must be other agencies that would 
prevent such a catastrophe. In fact, we have an abundance of examples 
of a reduction of enlarged capillaries whenever they occur. Thoma has 
given a satisfactory explanation of the closure of the ductus arteriosus and 
of the ductus venosus, but that is not quite to the point in this case. How- 
ever, in the beginning of the capillary system of the liver, around the 
omphalo-mesenteric vein, we have an excellent case. This question will 
be taken up at greater length subsequently, so at present I shall be very 
brief. The liver grows around and into the omphalo-mesenteric vein, and 
while so doing we have a double circulation, a more direct one through the 
constricted vein and a more circuitous one through the capillaries of the 
liver. But, in spite of this, the vein is gradually eliminated, leaving only 
a capillary plexus. The aortic arches of amphibia are eliminated in a 
similar way. From them loops of capillaries grow into the external gills 
which gradually take the place of the artery. There are numerous other 
examples. Minot,” who has recognized the fundamental importatice of 
the destruction of a main channel and its conversion into a system of 
capillaries, calls such vessels sinusoids, and the circulation through them 
a sinusoidal circulation. Not only are blood-vessels which are too large 
reduced, but it appears in the development of the blood-vessels in the tad- 
pole’s tail as if the new blood-vessels were always growing in the direction 
of the greatest resistance, for the nearest complete arch is already the 
shortest course from the artery to the vein; yet another and more distant 
one is to be formed. 
The first and guiding blood-vessel is the capillary which grows in all 
directions, forming a plexus. Secondary changes make arteries and 
veins of them and their laws of growth have been discovered and clearly 
stated by Thoma. The normal shape of the capillary is tubular with 4 
lumen about .008 mm. in diameter. They arrange themselves into a 
plexus with a tendency to come in contact with every surrounding cell. 
However, the tissues vary considerably in this respect, the first capillaries 
growing to them or past them in tufts. In general, the capillary ar- 
rangement is influenced by the tissue or organ into which it grows, but 
its conversion into main trunks and branches is controlled by the circu- 
2° See Oppel, Lehrbuch, III, p. 984. Brissaund and Sabourin are undoubtedly 
in error regarding this statement. I have made and examined hundreds of 
injections made with granules which would pass through small arteries but 
not through the capillaries, and have never seen such an anastomosis. 
Further, celloidin corrosions, which often include part of the capillaries of 
the lobule, never show such connections. 
* Minot, Proc. Bost. Soc. Nat. Hist., X XIX, 1900. 
