MINOT : SINUSOIDAL CIRCULATION. 
189 
distinctly sinusoidal. The figure shows here and there the vascular 
endothelium, endo, distinctly, and always fitted against the surfaces 
of the tubules and intertubular tissue strands. Whether or not the 
endothelium covers also the inner surface of the mesothelium, I 
have been unable to decide, for the wall of the pronephros is 
exceedingly thin. In the section the sinus is gorged with blood, 
which is omitted from the drawing for the sake of clearness. At 
the lower edge of the section a nephrostome or pronephric funnel 
Nst , is cut so as to show nearly its whole length ; it resembles that 
of Myxine, as figured by Semon, ’96.1 ; it is lined by very numer¬ 
ous long cilia, the presence of which is roughly indicated in the 
drawing; the cilia from the opposite sides overlap in the middle 
line and they are all bent inwards. The transition from the thin 
mesothelium to the cylinder epithelium of the funnel is abrupt. 
In a younger stage, Petromyzon fluviatilis of 24.6 mm., a section 
through the corresponding point is very similar to that figured 
above, — see Harvard Embryological Collection, No. 246, sections 
409-410. The morphological conditions are the same as in the 
older embryo, but the parts are smaller, the tubules being less in 
diameter, the blood spaces between them being less wide, and the 
ciliated funnel being both narrower and shorter. Also there are 
fewer pigment cells. In both these embryos, and in others of 
similar stages, the outside of each tubule when cut transversely 
shows a thin sharp line, which I consider the section of the endo¬ 
thelium of the blood space; here and there one finds a charac¬ 
teristic protuberant endothelial nucleus, Fig. 1, endo. I find 
nothing which corresponds to “ einer dllnnen Bindegewebshiille ” 
described by W. Muller except this endothelium. Probably Mfiller 
and myself have seen the same covering, and have given it differ¬ 
ent interpretations. W. Muller, l. c., p. 127, emphasizes the resem¬ 
blance of thp head kidney of the lamprey to that of the craniote 
anamnia, and I can fully confirm this statement, and add that it 
extends to the arrangement of the blood spaces in relation to the 
tubules. It may be added here that in an embryo of 7 mm. — see 
for example the Harvard Coll. No. 28, section 215 — the prone¬ 
phros is less than one tenth the size of that of Fig. 1 ; the funnel 
is perhaps one fifth as long; the tubules, nuclei, and blood spaces 
are all very much smaller than in Fig. 1. On the other hand, in 
older Ammocoetes, of 115-150 mm.— see Harvard Coll. No. 191, 
section 108, and No. 198, section 1438, — the nephrostomes and 
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