THE REGION OF THE SPINAL CORD 425 
be ascribed to the medullary part of the organ and to the separate 
paired organs in the Elasmobranchs, which are equivalent to the 
medullary part in other cases. 
The evidence, then, of the transformation of the known vertebrate 
excretory organs—the pronephros and the mesonephros—leads to the 
conclusion that in our search for the missing coxal glands of the 
meso- and pro-somatic regions, we must look for either lymphatic 
glands, or ductless glands of distinct importance to the body. I have 
already considered the question in the prosomatic region, and have 
given my reasons why the pituitary gland must be looked upon as 
the descendant of the arthropod coxal gland. In this case also the 
resulting ductless gland is still of functional importance, for disease of 
it is associated with acromegaly. If, as is possible, it is homologous 
with the Ascidian hypophysial gland, then it is confirmatory evidence 
that this latter is said by Julin to be an altered nephridial organ. 
Finally, I come to the mesosomatic or branchial region ; and here, 
strikingly enough, we find a perfectly segmental glandular organ of 
mysterious origin—the thymus gland—-segmental with the branchie, 
not necessarily with the myotomes, belonging, therefore, to the appen- 
dicular system ; and since the branchie represent, according to my 
theory, the basal part of the appendage, such segmental glands would 
be in the position of coxal glands. Here, then, in the thymus may 
be the missing mesosomatic coxal glands. 
What, then, is the thymus ? 
The answer to this question has been given recently by Beard, 
who strongly confirms Kélliker’s original view that the thymus is a 
gland for the manufacture of leucocytes, and that such leucocytes are 
directly derived from the epithelial cells of the thymus. Kolliker 
also further pointed out that the blood of the embryo is for a certain 
period destitute of leucocytes. Beard confirms this last statement, 
and says that up to a certain stage (varying from 10 to 16 mm. in 
length of the embryo) the embryos of Raja batis have no leucocytes 
in the blood or elsewhere. Up to this period the thymus-placode is 
well formed, and the first leucocytes can be seen to be formed in it 
from its epithelial cells; then such formation takes place with great 
rapidity, and soon an enormous discharge of leucocytes occurs from 
the thymus into the tissue-spaces and blood. He therefore concludes 
that all lymphoid tissues in the body arise originally from the thymus 
gland, z.e. from leucocytes discharged from the thymus. 
