THE REGION OF THE SPINAL CORD 427 
the pleural fold with the branchial body-surface, then the remains of 
the position of the atrial chamber must exist in Ammoccetes as that 
extraordinary space between the somatic muscles and the branchial 
basket-work filled with blood-spaces and modified muco-cartilage. It 
is in this very space, close against the gill-slits, that the thymus 
glands of Ammoccetes are found, in the very place where the nephric 
tubules of Amphioxus would be found if its atrial cavity were closed 
completely. Instead, therefore, of considering with Boveri that the 
branchial nephric tubules of Amphioxus still exist in the Craniota 
as the pronephros, and that the atrial chamber has narrowed down to 
the pronephric duct, I would agree with van Wijhe that the pro- 
nephros is post-branchial, and suggest that by the complete closure 
of the atrial space in the branchial region the branchial nephric 
tubules have lost all external opening, and consequently, as in all 
other cases, have changed into lymphatic tissue and become the 
segmental thymus glands. 
As van Wijhe himself remarks, the time is hardly ripe for making 
any positive statement about the relationship between the thymus 
gland and branchial excretory organs. There is at present not suffi- 
cient consensus of opinion to enable us to speak with any certainty 
on the subject, yet there is so much suggestiveness in the various 
statements of different authors as to make it worth while to consider 
the question briefly. 
On the one hand, thymus, tonsils, parathyroids, epithelial cell- 
nests, and parathymus, are all stated to be derivatives of the epithelium 
lining the gill-slits, and Maurer would draw a distinction between 
the organs derived from the dorsal side of the gill-cleft and those 
derived from the ventral side—the former being thymus, the latter 
forming the epithelial cell-nests, i.e. parathyroids. The thymus in 
Ammoccetes, according to Schaffer, lies both ventral and dorsal to the 
gill-cleft ; Maurer thinks that only the dorsal part corresponds to 
the thymus, the ventral part corresponding to the parathyroids, etc. 
Structurally, the thymus, parathyroids, and the epithelial cell-nests 
are remarkably similar, so that the evidence appears to point to the 
conclusion that, in the neighbourhood of the gill-slits, segmentally 
arranged organs of a lymphatic character are situated, which give 
origin to the thymus, parathyroids, tonsils, etc. Now, among these 
organs, i.e. among those ventrally situated, Maurer places the 
carotid gland, so that, if he is right, the origin of the carotid gland 
