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 branchice, 

 not necessarily with the myotomes, belonging, therefore, to the appen- 

 dicular system ; and since the branchiae 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 Kolliker'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, i.e. from leucocytes discharged from the thymus. 



