306 
MR. H. GRAY ON THE DEVELOPMENT OF THE 
on each side of the trachea. The latest researches on the development of the thyroid, 
appear to be those of Mr. Goodsir, who states that it is, like the thymus and supra- 
renal gland, a development from the “ membrana intermedia.” 
According to my observations, between the sixth and seventh days of incubation, 
the first trace of the thyroid gland becomes apparent. It consists of an exceedingly 
small spherical whitish mass of blastema, situated on each side of the root of the 
neck, close to the point where, at a later period, the carotid and subclavian arteries 
separate from one another. The outer margin of each mass is somewhat irregular 
and ill-defined, and is not apparently surrounded by any investing membrane ; above 
each of them is seen the lower end of the rudimentary thymic tube, the outer margins 
of which are perfectly distinct from the thyroid. The carotid vessel runs beneath 
and on the outer side of the gland, to the wall of which it is somewhat adherent ; and 
on the inner side runs the rudimentary tracheal tube, with which however it has no 
connection. The structure of the glands at this period approximate very closely to 
that of the spleen and supra-renal gland at the earliest stages of their evolution, con- 
sisting of granular matter, nuclei, and nucleated vesicles. The granular matter, 
which forms a large mass of the substance of the gland, is made up of innumerable 
minute pale granules, very similar to those observed in the structure of the spleen, 
but unlike those of the supra-renal glands, the particles composing which are darker 
and more refractive, not unlike minute fat granules. The nuclei are about the size 
of the red blood-discs ; they are pale, perfectly circular, and contain in their interior 
from one to four or five small dark granules. These also form a very considerable 
portion of the structure of the organ. The vesicles are few in number, pale, and 
delicate, with nuclei on their walls, and containing in their interior a few fine dark 
granules. On the eighth day of incubation the organ has enlarged very slightly, 
occupying the same position as on the previous day; its colour is now reddish white, 
from the presence of blood in its structure ; it is circular, and is bounded externally 
by a clearly defined marginal membrane, which now surrounds it. I have never been 
able, in tracing out the incipient development of these organs, to detect any connec- 
tion between it and the branchial cleft, the trachea, or the thymus, although its 
position as regards these parts has probably given rise to the opinions, previously 
noticed, respecting the origin or connection of the thyroid from one or other of these 
structures. It has always appeared to me to arise as a separate mass of blastema, and 
unconnected, unless by simple apposition, with any other organ. From this period, 
up to the tenth day of incubation, the thyroid, with the exception of a slight enlarge- 
ment, and a more distinct red colour, presents the same structure, and occupies a 
similar position as in the previous examinations. At this period they are found situ- 
ated a little above the root of the neck, one on each side of the lower end of the 
trachea, just above its division into the bronchi, and at the point of origin of the 
carotid vessel. The organ is now more distinct from its reddish tinge ; its form 
is circular, its outer margin clear and distinct ; and its substance, when seen with a 
