11C 2 
DR. H. WATNEY ON THE MINUTE ANATOMY OE THE THYMUS. 
appearance from the epithelial cells of the thyroid, which lie in approximation to the 
upper part of the tube. The lumen of the tube is in some cases closely packed with 
blood corpuscles; and appears to he continuous with vessels which pass out through 
the epithelioid cells. 
In the foetal Rabbit eighteen days old there are present in the neck three glands : 
first, the thyroid; secondly, a gland similar in appearance to the thyroid, and lying on 
its outer side, and in front and to the inner side of the carotid artery ; this gland does 
not extend so high up in the neck as the upper part of the thyroid, but reaches down 
nearly to the upper end of the thymus ; and in the embryo of twenty days, it extends 
still further, and lies behind and to the outer side of the upper end of the thymus. 
We also find in embryoes eighteen days old, thirdly, the thymus, wdiich in the upper 
part is composed of a tube or tubes, and in the lower part is more branched. The 
upper part of the thymus retains its foetal form much more than the lower part, as was 
noticed by Kolliker (76). Vessels and some connective-tissue are seen to pass into 
the follicle, and the outer cells forming the border of the follicle are more epithelioid 
in character than the inner cells. The growth of the thymus can be easily traced in 
specimens of this age; and it will be seen that long processes of epithelioid cells are 
pushed out in the surrounding connective-tissue (see Plate 95, fig. 109, a). 
In the foetal Rabbit of twenty-one or twenty-two days old the cells are much 
smaller, and do not resemble epithelioid cells, except at quite the upper part of the 
gland. In the embryo of thirty days old the cells are still smaller, and the thymus 
nearly resembles that of newly-born animals. 
In sections of the Chick, early in the seventh day of incubation, two small circular 
collections of cells are found, behind, and to the outer side of the nerve and vein of the 
neck (see Plate 87, fig. 29). The first growth is the enlargement of the gland, pushing 
out the wall in projections, similar to those in Plate 87, fig. 23. This can be well seen 
on the eleventh day. On the fourteenth day the connective-tissue has invaded the 
gland, and divided it into separate follicles ; and on the eighteenth day these are 
differentiated into cortex and medulla. The large epithelioid cells are not seen until 
the cortex has been differentiated from the medulla. 
It will be seen from the above that we do not accept the epithelial origin of the 
thymus. 
If, however, we accepted the views of those authors who conclude that the thymus 
arises from epithelial cells, it appears to me that the view of Kolliker (76) must be 
maintained, and that the opinion of Stieda (53) is incorrect. The latter author 
considers that the epithelium forms the concentric corpuscles, and that the lymphoid 
tissue is formed from the middle layer which invades the gland ; if, however, sections 
of the thymus are made, at various stages of growth, it will be seen that the epithelioid 
cells become smaller and smaller, and that they are pressed to the outside of the follicles, 
while the connective-tissue and blood vessels invade the interior of the follicle ; and it 
is certain that the concentric corpuscles are found chiefly in the interior of the follicle, 
