DUCTLESS GLANDS IN THE CHICK. 
307 
low magnifying power, moderately transparent. Its minute structure also now pre- 
sents a higher stage of development ; for besides consisting, as at an earlier period, 
of granular matter, nuclei and vesicles, some of which are furnished with nuclei, 
there are observed numerous circular and tolerably transparent masses of nuclei 
and vesicles, some perfectly destitute of any investing membrane, but others pre- 
senting a fine membrane partially or completely investing them. In the interspaces 
between these vesicles numerous blood-discs may be observed, which are apparently 
not as yet contained in separate tubes. 
On the twelfth day, the size of the thyroid about equals that of a millet-seed ; it is 
bright red, and small vessels may now be traced passing into its substance. Its 
minute structure, however, undergoes no change until about the sixteenth day. At 
this period a delicate membrane, forming a complete capsule, can be removed from 
its external surface ; it consists of numerous nucleated fibrillm. Almost the entire 
mass of the organ is composed of large circular or oval vesicles, consisting of a mass 
of nuclei, enclosed in a faintly delicate, homogeneous, limitary membrane. On the 
twenty-first day the thyroid is rather smaller than the spleen, and rather larger than 
the supra- renal glands. It is of a reddish gray colour, and is surrounded by a com- 
plete fibrous capsule, which may be easily removed from its exterior. It consists of 
a mass of vesicles, which vary in size ; they are chiefly circular, and their contents 
transparent. They consist of an external, homogeneous and transparent membrane, 
forming a closed cavity, which contains a mass of nuclei. In some cases, however, 
the vesicles, instead of being filled with nuclei, are lined with a layer of nucleated 
cells, and a cavity, although small, exists in their interior. 
It may be seen, from the preceding observations, that the thyroid glands are deve- 
loped in the form of two distinct separate masses of blastema, one at each side of the 
root of the neck, close to the point of separation of the carotid and subclavian 
vessels, and between the trachea and the bronchial clefts, but quite independent, as 
far as regards their evolution, of either of those parts. Their minute structure also, 
at an early period of their development, closely corresponds both with the spleen and 
supra-renal glands ; and the tissues of which they are composed, at a later period, are 
formed in a n)anner precisely similar with the same parts in those organs, a fact which 
shows the analogy they bear to one another. 
From the preceding observations, it will be seen that a close analogy exists between 
the glands already described ; and the propriety of their classification, together with 
the thymus, under one group, as the “ Ductless Glands,” may be considered clearly 
proved. Now although most anatomists, excepting Mr. Goodsir in this country, 
adopt such a classification, and place the spleen under the head of the “ductless 
glands,” many of our continental anatomists, among whom may be enumerated Ecker, 
in a very late and highly elaborate article, has attempted to prove that this organ 
cannot, either anatomically or physiologically, be enumerated with them. “For,” 
says Ecker, “ though the vesicles of the spleen have a similar function with the glands 
