212 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 
Physiology of sinusoids. On this subject I have only to 
point out the obvious fact that the physiological processes connected 
with the blood must occur in sinusoids under conditions very differ¬ 
ent indeed from those in capillaries. The recognition of sinusoids 
may be considered as equally important to the anatomist and 
physiologist. 
Phylogenetic. In the embryo the sinusoids and true capillaries 
begin their development about the same time, the capillaries 
perhaps being the earlier. We cannot therefore assume precedence 
for one over the other in the evolution of vertebrates. We find in 
fact that when the sinusoids of the liver and pronephros appear, 
the capillaries of the sub-ectodermal coat are already present. This 
sub-ectodermal layer has not been recognized hitherto; it may be 
called the primitive vascular tunic or panchoroidea of vertebrates. 
On another occasion I hope to publish an account of it. 
In the course of the evolution of vertebrates the sinusoids become 
less important. As regards the four principal organs with sinusoids, 
the pronephros with very wide sinusoids gives way to the meso¬ 
nephros with narrower sinusoids, and the latter gives way to the 
kidney with true capillaries. The liver has its sinusoids rapidly 
transformed into capilliform vessels. In the heart with the addition 
of the coronary arteries the sinusoidal circulation is at least supple¬ 
mented by the capillary. In all these cases the higher forms have 
vessels of smaller calibre substituted for larger ones directly supply¬ 
ing the blood needs of the tissues. This reduction in size is another 
example of a well-known physiological principle, of which other 
illustrations are offered us by the progressive reduction of size in 
the blood corpuscles as we ascend the vertebrate series, and by the 
reduction of size in the excretory tubules as we pass from pro¬ 
nephros to mesonephros, and then on to the kidney proper. Thus 
we may assume that the physiological inferiority of the sinusoidal 
type of circulation has led to its modification and partial obliteration 
in the higher vertebrates. 
Harvard Embryological Laboratory, 
January, 1900. 
