1100 
DR. H. WATNEY ON THE MINUTE ANATOMY OE THE THYMUS. 
much resembles that of tbe Tortoise (see Plate 95, fig. 105). In the medulla there are 
a few concentric corpuscles formed of epithelioid cells. 
In the thymus there is a follicle composed entirely of flattened, spindle-shaped cells; 
this exactly resembles those which are met with in the Tortoise, and which have been 
supposed by Afanassiew (33) to be individual follicles, undergoing involution before 
the rest of the gland ; there is no evidence that this is the case ; on the contrary, all 
the facts point to its being some other gland which is situated among the thymic 
follicles. As in the first place, it is found in quite young Reptiles, as in the small 
Tortoise and the young Alligator; and in Mammals and the other classes of Vertebrates 
we do not find individual follicles undergoing involution before the rest of the gland. 
Secondly, when a follicle undergoes involution the process is a very gradual one, and is 
never characterised by a change of the whole follicle, equally and everywhere, into one 
tissue. Thirdly, these follicles are composed everywhere of flattened, spindle-shaped 
cells, and no such tissue is met with in the undoubted thymic follicles of either old or 
young Reptiles. Fourthly, no such tissue is met with in the thymus of Fish, Amphibia, 
or Birds; exactly such follicles are, however, found in Mammals, where hibernating 
glands exist, as in the Hedgehog. We conclude that these follicles are those of some 
other gland, probably of the nature of the hibernating gland of Mammals. 
In the large Snake the thymus is small and to a great extent transformed into 
fibrous-tissue. The follicles contain many concentric corpuscles of large size, and a 
large number of granular cells, and the steps of transition between granular cells and 
fibrous-tissue are well seen. 
This gland also contains a follicle, composed entirely of flattened, spindle-shaped cells, 
such as we have just spoken of, and there is no resemblance between this follicle and 
the thymic follicles which are undergoing involution by the invasion and new formation 
of fibrous tissue. 
Thymus of Amphibia. 
In the Frog the medulla contains many granular cells, as many as forty to sixty can 
often be seen in a section of the small organ. The granular cells are clup-shaped, and 
are attached by their smaller ends to fibrous tissue. 
The thymus of the Axolotl is situated in nearly the same position as in the Ray-fish. 
It often contains cysts of considerable size. Its cells are very large. 
Thymus of Fish. 
In the Ray-fish the thymus begins as one follicle (see Plate 84, fig. 5), which is 
afterwards divided into numerous follicles. These are, in the later stages of involution, 
widely separated from one another. The thymus is very soft, and requires hardening 
with great care, lest in making sections the greater part of the lymphoid tissue should 
be lost. 
