DUCTLESS GLANDS IN THE CHICK. 
305 
are of a deep yellow colour ; their position is now almost precisely the same as in the 
adult bird; numerous small vessels ramify upon their surface, and a large vein emerges 
from their inner sides, which empties itself into the vena cava. A complete division 
into cortical and medullary portions is now observed. The former consists of elon- 
gated tubes with exceedingly delicate walls, and radiating from the circumference 
towards the centre of the gland. These tubes, which at this period exist in greater 
number, are parallel with one another, and lie between a close mesh of delicate capil- 
lary vessels, having a similar direction. In some cases the simple vesicles placed end 
to end, but not as yet forming tubes, are disposed in a similar manner between the 
capillary vessels. The medullary portion of the gland consists of capillaries, which 
here join to form larger branches, either previous to their passage from the gland, or 
before passing into the cortical portion in the form of straight capillaries; numerous 
delicate fibres may also be observed joining them together, and forming a close mesh, 
in which are deposited numerous dotted corpuscles, and a quantity of fine dark granules. 
From this time, up to the completion of incubation, no change takes place in the 
structure of these glands. From the preceding observations it is seen that the supra- 
renal glands are developed by two separate masses of blastema, which are situated 
between the upper and inner extremities of the Wolffian bodies and the sides of the 
aorta, but which are totally independent (as far as concerns their development) of 
those bodies, or of each other. It is also seen that their minute structure at an 
early period closely corresponds with the structure of the spleen ; and although the 
supra-renal glands attain their maximum degree of development at an earlier period 
than that organ, as regards the formation of its vesicles, still exactly the same pro- 
cess can be followed in the spleen as regards the development of the Malpighian 
vesicles ; a fact which, I think, tends to prove the great similarity of the organs in 
question. 
Development of the Thyroid Glands. 
The evolution of the thyroid gland, like the spleen and supra-renal capsules, is in- 
volved in great obscurity. 
The following are the principal accounts of its development that have yet been 
given. r 
According to Huschke* the thyroid gland is developed from the anterior branchial 
arches ; no other observer however has verified his statement, and Rathke altogether 
denies it. 
Arnold'!' states that it is developed from the membranous air-tube, in the situation 
where the larynx is formed, and that it is at first provided with an excretory duct. 
Bischoff;!: states that they appear from a single formative mass, which is deposited 
* Huschke, Isis, p. 621, 1826, p. 403, 1827. 
t Arnold, Lehrbuch der Physiologic des Menschen. Zurich, 1842, vol. v. p, 1293. 
J Th. L. Bischoff, Entwick. der Saiig. und des Mens. Leipzig, 1842. 
2 R 2 
