MACDONNELL ON ANIMAL DEXTRINE. 
515 
named it, does not show itself in any tissue (except perhaps in the 
muscles which move the branchial arches) up to the moment of 
the appearance of the posterior limbs. It then makes its appearance 
in the cartilage of ossification of these limbs. In the embryo of the 
chick the cartilaginous tissue is also the first in which it shows itself, 
and up to the fourteenth day of incubation it is not to be met with 
m any other tissue, except the horny membrane of the bill. In an 
embryo of a sheep of about two months, the cells of the cartilage of 
ossification, and those of the trachea showed in contact with iodine 
a colour absolutely like that of the horn cells. The muscular 
elements of animal life contain also at this epoch a considerable 
quantity of the amylaceous plasma. These elements have at this 
period the form of tubes, and are composed of a membranous wall, 
of a cortical covering and of a central canal. The surcolimma is not 
sensibly coloured; the cortical covering, formed by the peculiar granu¬ 
lations of the muscular substance, is only coloured yellow, like all 
the azotised matters. Within the central canal neither the nuclei 
nor the granulations (generally of a fatty nature) take any peculiar 
colour; the violet tint is exclusively confined to the liquid, homo¬ 
genous, amylaceous plasma, which fills the central canal. 
It is, I believe, to the condition and properties of the amyloid 
substance as met with in young and rapidly growing tissues, that we 
must chiefly look for a full explanation of the true function performed 
in the animal economy by this widely diffused ingredient of the 
organism. There is still an extensive field here open to physiologists, 
and one which will doubtless amply repay labour expended upon it. 
In concluding this resume I may briefly enumerate the principal 
facts at present known concerning the existence of the amyloid 
substance in the foetal tissues. Any person who takes the trouble 
to examine the tissues of a mammalian foetus will be at first surprised 
with the large amount of starch-like matter with which they are 
impregnated. Nor is this confined to mammalia; indeed it would 
appear that there is a particular moment in their growth when this 
amyloid substance seems to be a formative matter from which many 
tissues are evolved ; that, in fact it is related to their growth and 
development as starch is to the growth and development of vegetable 
tissues. Let a small portion of the skin of a chick in ovo, at the 
moment when the epithelial cells have arranged themselves in the 
little masses which indicate the commencing development of the 
feathers, be placed under the microscope, and let some acidulated 
tincture of iodine be added, the peculiar brown colour will at once 
show every cell to be filled with amylacious matter, making a striking 
object. In the same way it may be demonstrated in the skin of the 
embryos of rabbits, cats, guinea pigs, sheep, oxen, pigs,&c. as also most 
abundant at the points where the aggregation of epithelial cells shows 
that the hairs are about to spring. Even more beautifully it may be 
seen in the denser horny structures; in the bill of the chick, the 
claws of the kitten, or the hoof of the foetal calf; and from the 
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