MR. E. C. BABER ON THE STRUCTURE OF THE THYROID GLAND. 583 
always be cut across. W. Muller ( loc. cit.), wlio appears to agree with the majority 
of observers in considering that in the fully developed state the vesicles are closed 
globular bodies, describes an earlier stage (in the development of the gland) in which, 
before the formation of vesicles the gland consists of a network of cylindrical tubes, 
at first irregular, but afterwards becoming more regular in character. They are 
composed of a very fine envelope of connective tissue, and are filled in the interior 
with short cylindrical epithelial cells, placed radially on the envelope. The tubes are 
separated by processes of the mesoblast, which have grown inwards from the con¬ 
nective tissue surrounding the gland. The tubes are at first solid, but are subsequently 
provided with a central lumen. After the lumen in the interior of the tubes has 
become more marked, the separation of different segments of the tubes takes place by 
the growth into them laterally of processes of the mesoblast, which results in the 
formation of gland-vesicles. The same observer finds that in the human thyroid the 
shape of the vesicles varies much from the fifth month of embryonic life to the age of 
three years. From this period to puberty the changes occurring are, he considers, 
that the number of glandular deposits continues to diminish and the follicles 
increase in size, by the accumulation of secretion, which shows a great tendency to 
become thickened, and they approach the spherical shape. Peremeschko (op. cit., 
p. 283) found the vesicles in young embryos regularly round ; in mature embryos 
mostly of irregularly polygonal form ; in young animals very seldom of regular round 
form, and still more rarely so in adult animals. In the latter the vesicles have, he 
says, the most different forms ; they are almost always polygonally round or poly¬ 
gon ally oval. 
Observations. 
My own observations on this subject may be summarised as follows :— 
Dog .—In the gland of young Dogs (aged five weeks and three months respectively'") 
I have observed that the gland-vesicles are very much branched and present numerous 
hollow ramifications (see Plate 68, figs. 2 and 3). In order to see these well in 
sections of hardened glands it is necessary that the sections should be thick, and then 
by careful focussing, the vesicles with their usual contents can be traced ramifying both 
upwards and downwards, and laterally in all directions. In the glands of numerous 
other Dogs of different ages I have ascertained with equal certainty that the vesicles 
present very few, if any, ramifications. When the vesicles present these branches, 
the intervals between them form the inflexions projecting into the interior of the 
vesicles, which have been already described by Verson (op. cit., p. 371) and myself 
(op. cit., p. 560). I have examined fresh portions of the glands of young Sheep in salt 
solution and in Muller’s fluid (according to the method recommended by Zeiss for 
floating out the tubes), but have obtained no appearances that might not be produced 
by the much-branched vesicles just described in the thyroid glands of young Dogs. In 
fact, there can be no doubt that these are the same structures as those which Zeiss 
* Tlie ages of most of tlie Dogs mentioned in this paper were only approximately ascertained. 
