THYMUS AND THYROID BODIES. 
639 
and external portion of the intestinal canal, is found massed along- the trunks of the 
primitive venous system, the sides of the arches of the aorta, the terminal portion of 
that vessel, and the origins of the omphalo-mesenteric arteries. 
The portions of the membrana intermedia which are last of being converted into 
special organs, the Wolffian bodies, are the parts which project one on each side 
of the aorta, along the posterior part of the cardinal veins of Rathke, between the 
intestinal plates and visceral laminae. 
The portions of the membrana intermedia which remain between the upper ex- 
tremities of the Wolffian bodies, and the heart and liver, and which surround the 
origins of the omphalo-mesenteric arteries, do not become converted into organs of 
special structure, but retain during life the original constitution of the membrana 
intermedia of the blastoderma, and increase rapidly in the embryo, constituting the 
supra-renal capsules. Whatever doubt may be entertained as to the exact functional 
import of these bodies, the identity of their anatomical constitution with that of the 
blastoderma is sufficiently evident, and their morphological signification appears to 
be equally so. 
That portion of the membrana intermedia which is situated between those two 
aortic arches, the extremities of w r hich become the carotid and subclavian arteries, 
remains during life as the thyroid body. It receives its blood from the first and 
second aortic arches by two large trunks on each side, the superior and inferior thyroid 
arteries. 
That portion of the membrane which passes in two parts from near the base of the 
cranium back as far as the ductus Cuvieri and anterior portions of the veins of 
Rathke, and which are united and concentrated in front of the heart by passing 
from behind forwards, in harmony with corresponding motions of the neighbouring 
part, becomes the thymus. 
The structure of these three organs is identical with that of the blastoderma. Their 
probable function, namely, to prepare by the action of their nucleated cells, and to 
throw into the vascular system a matter necessary for the nutrition of the animal 
during the period of its active growth — a function which the observations and opi- 
nions of the majority of physiologists have assigned to them — is also essentially the 
same with that of the blastoderma. 
The question as to the exact or intimate nature of the function of these organs can 
only be answered by further inquiries in chemical physiology. It appears to me to 
be sufficient at present to insist, that their functions, as deduced from their struc- 
ture and anatomical relations, is similar to that performed by the blastoderma, what- 
ever the exact nature of that function may be. 
I have therefore been led to consider the supra-renal capsules, the thymus and 
thyroid, as organs essentially similar in structure ; as developments of the remains of 
the blastoderma, being formed of a continuous portion of that part situated along 
each side of the spine, from the Wolffian bodies to the base of the cranium, the supra- 
