DR. H. WATNEY ON THE MINUTE ANATOMY OF THE THYMUS. 
1101 
Many colourless blood corpuscles are found in the connective-tissue and in the 
follicles. They are easily distinguished from the lymphoid cells by their granular 
appearance and greater size. The Pay fishes used in this research (with the exception 
of the smaller ones) had all been dead about ten or twelve hours before they were pro¬ 
cured. The presence of colourless corpuscles in the tissues may possibly be only due 
to wandering of colourless corpuscles from the vessels after the death of the animal. 
The cortical tissue does not differ from that of Mammals, and is represented in 
Plate 95, fig. 103. During involution there are great changes in the vessels; they 
become surrounded by one or more perivascular sheaths. 
The tissue found in the Cod-fish, by cutting through the lining membrane of the 
upper and back part of the gill cavity, is much harder than the thymus of the Pay- 
fish. The whole tissue is encapsulated with connective-tissue, from which proceed 
trabeculae, dividing the lymphoid masses into definite follicles, of which the outer are 
larger than the inner. 
The lymphoid tissue does not show any differentiation between cortex and medulla, 
nor any central ring of vessels; and the trabeculae are covered on each side by 
endothelial cells. 
This tissue is probably a lymphatic gland. 
Development. 
In the History (p. 1073) an account of the various views which have been held 
concerning the development of the thymus has been given. On the one hand are 
the observations of Fuiedleben (3) and Afanassiew (33), who describe the thymus 
as arising independently in connective-tissue ; and on the other hand, the observations 
of the authors who believe that the thymus arises from the respiratory tract, or the 
pharynx, or from the branchial clefts. 
The development has been traced in the thymus of the human embryo, and in that 
of the Pabbit and the Chick. 
In the human embryo about two and a-half inches long, it is found in the thorax and 
the neck, as a double gland; the upper part of each half consists of a single follicle, 
not differentiated into cortex and medulla. The wall of the gland has projections on 
its exterior (see Plate 87, fig. 23), and the connective-tissue has slightly invaded the 
gland. 
In the human embryo twelve weeks old, the thymus consists of several follicles, 
partially differentiated into cortex and medulla; it also contains a few concentric 
corpuscles (see Plate 86, fig. 11). 
In the foetal Pabbit sixteen days old, we find the thymus in the neck, lying in front 
and to the inner side of the carotid artery. It is seen, at its upper end, as a single 
tube, and below as a number of tubes, some of which are solid and others hollow ; 
the hollows are of very different sizes. The cells in cross-sections of the tube vary 
from two or three to many in number; they are epithelioid in character, and differ in 
