MINOT : SINUSOIDAL CIRCULATION. 
199 
which I hold to be of capital importance, appears to have been 
overlooked by Balfour, probably because the preservation of his 
specimens did not permit its recognition. In embryos of 13 mm. 
the hepatic cylinders have grown, and everywhere between them 
are typical sinusoids. In embryos of 18 mm., of which we have 
better preparations, the sinusoids occupy the entire room between 
the cylinders, and the endothelium follows everywhere the curves 
of the cylinders; there is a little mesenchyma at certain points 
under the covering mesothelium and around the entoderm of the 
main diverticula, but I have 
been unable to make sure of 
any mesenchyma, except the 
endothelium between the cylin¬ 
ders— compare Harvard Coll., 
Embryo 203, section 475 et al. 
The condition in embryos of 29 
mm. I have already described 
in my “ Human Embryology.” 
For convenience I reproduce 
the figure and quote: “Every 
cylinder, hp, is an epithelial 
tube with a small central lumen 
and covered by an endothelium, 
which is easily recognized by 
its flattened darkly stained nu¬ 
clei ; the endothelium is the 
wall of the blood vessel or 
channel, pi. The hepatic cylin¬ 
ders by branching and uniting form a network, all the meshes of 
which are entirely occupied by blood vessels.” So far as I am 
aware the true character of the hepatic circulation in elasmobranclis 
was first pointed out in this passage (1892). In an embryo of 34.0 
mm. the reduction of sinusoids in size is marked, and they are 
more like separate vessels owing to the more frequent unions of 
the cylinders, so that we have a distinct approach towards the adult 
condition as described by Hermann Brans, ’96.1, p. 315. The 
reduction is much more marked in the cephalad than in the caudad 
end of the liver, and in the latter indeed the sinusoids are still 
relatively wide — compare Harvard Coll., No. 202, slides T, TJ and 
Y. The retarded development of the caudad tip of the liver as 
Fig. 6. Squalus acanthias. Portion 
of a section of the liver of an embryo 
of 29 mm.; hp, hepatic cylinders ; bl , 
blood sinusoids. 
