426 THE ORIGIN OF VERTEBRATES 



The segmental branchial glands, known by the name of thymus, 

 are, according to this view, the original lymphatic glands of the 

 vertebrate ; and it is to be noted that, in fishes and in Amphibia, 

 lymphatic glands, such as we know them in the higher mammals, do 

 not exist ; they are characteristic of the higher stages of vertebrate 

 evolution. In the lower vertebrates, the only glandular masses 

 apart from the cell-lining of the body-cavity itself, which give rise 

 to leucocyte-forming tissue, are these segmental branchial glands, or 

 possibly also the modified post-branchial segmental glands, known as 

 the head-kidney in Teleostea, etc. 



The importance ascribed by Beard to the thymus in the forma- 

 tion of leucocytes in the lowest vertebrates would be considerably 

 reduced in value if the branchial region of Ammoccetes possessed 

 neither thymus glands nor anything equivalent to them. Such, 

 however, is not the case. Schaffer has shown that in the young 

 Ammoccetes masses of lymphatic glandular tissue are found segmen- 

 tary arranged in the neighbourhood of each gill-slit — tissue which 

 soon becomes converted into a swarming mass of leucocytes, and 

 shows by its staining, etc., how different it is from a blood-space. 

 The presence of this thymus leucocyte-forming tissue, as described 

 by Schaffer, is confirmed by Beard, and I myself have seen the same 

 thing in my youngest specimen of Ammoccetes. 



Further, the very methods by which Kowalewsky has brought 

 to light the segmental lymph-glands of the branchial region of the 

 Crustacea, etc., are the same as those by which Weiss discovered the 

 branchial nephric glands in Amphioxus — excretory organs which 

 Boveri considers to represent the pronephros of the Craniota. In 

 this supposition Boveri is right, in so far that both pronephros and 

 the tubules in Amphioxus belong to the same system of excretory 

 organs; but I entirely agree with van Wijhe that the region in 

 Amphioxus is wrong. The tubules in Amphioxus ought to be repre- 

 sented in the branchial region of the Craniota, not in the post- 

 branchial region ; van Wijhe therefore suggests that further researches 

 may homologize them with the thymus gland in the Craniota, not 

 with the pronephros. This suggestion of van Wijhe appears to me 

 a remarkably good one, especially in view of the position of the 

 thymus glands in Ammoccetes and the nephric branchial glands 

 in Amphioxus. If, as I have pointed out, the atrial cavity of 

 Amphioxus has been closed in Ammoccetes by the apposition of 



