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- 
tally 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 Wijbe 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 Ammocctes by the apposition of 
