12 
DR. C. H. JONES ON THE STRUCTURE, DEVELOPMENT 
ring yolk-cells, and overspread with ramifications of the omphalo-meseraic vessels. 
When the constriction of the germinal membrane takes place, two tracts of oily 
matter appear, one passing forwards and the other backwards from the constricted 
part, where they are continuous with each other. The margins of these tracts are 
quite even, and soon become invested with a homogeneous membrane, but at first 
I doubt if this is actually present ; in a chick at the seventy-seventh hour of incubation, 
the transparent membrane of the vitelline duct appeared to me to lose itself upon tlie 
tracts of oily matter. The upper margin of the vitelline duct and of the oily tracts 
forms at first nearly a straight line, which is separated by a considerable space from 
the vertebral column, one-half to one-third of this space being occupied by the Wolf- 
fian bodies. The posterior tract ceases near the caudal extremity ; it is, I think, rather 
thicker but shorter than the anterior. This, in the earliest periods after passing for- 
ward with a slight curve, is lost just as it reaches a quantity of blastema situated 
close behind the heart. This blastema is the rudiment of the parenchyma of the 
liver; it does not consist of oily matter like that of the intestine, but of a less opake 
and more granular substance. As development advances the anterior tract continues 
to extend ; it passes forwards and upwards above and behind the liver, and was traced 
on one occasion, though becoming more faint, as far as the last branchial fissure ; a 
slight dilatation at one part of its course marks the future stomach, and from near 
the same part two offsets are given off, one of which runs upon the liver, the other 
towards the vertebral column. Both oily tracts and the offsets just mentioned are 
solid ; they are to be regarded, I think, as a peculiar deposit of formative matter in- 
tended for the formation of particular structures ; their appearance and mode of 
development seem to me by no means consonant with the view that the intestinal 
cavity is simply a part shut off from the general yolk cavity. In fact, the intestine, 
as has been said, is not at first a cavity but a solid tract of formative matter, and in 
this respect is analogous to the embryonic condition of other organs (so far as I can 
determine), which always seem to arise from a shapeless blastema deposited in one 
spot. The offset to the liver is more distinct sometimes than at others ; it is most 
marked about the sixth day, but always may be distinguished as a small eminence of 
opake oily matter in the side of the intestine, until the cystic and hepatic ducts are 
fully formed and have united with the intestine at that spot. The existence of this 
eminence is certainly very remarkable ; it may be well to give it a name and call it 
the colliculus I I do not see that its import at present can be at all explained; nor 
could we indeed expect this, unless we could obtain some clue to the comprehension 
of the mysterious principle by wdiich the process of development is governed. I can- 
not see any ground for the statement that the liver is developed by the protrusion of 
the walls of the intestinal canal ; it certainly appears at a very early period as a 
parenchymatous mass of blastema ; and even if one could grant that this blastema 
originated in any way from the anterior oily tract which passes close to it, still it 
remains, according to my observation, perfectly unquestionable, that after the in- 
