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 consecpuently, 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 

 Ammocujtes, 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 



