THE EVIDENCE OF THE THYROID GLAND 1 99 



from a degenerated Selachian ; therefore, in order to be logical, he 

 ought to show that the thyroid of Ammoccetes is an intermediate down- 

 ward step between the thyroid of Selachians and that of Amphioxus 

 and the Tunicates. Here, it seems to me, his argument utterly breaks 

 down ; it is so clear that the thyroid of Petromyzon links on to that 

 of the higher fishes, and that the Ammocoetes thyroid is so immeasur- 

 ably more complicated and elaborate a structure than is that of 

 Petromyzon, as to make it impossible to believe that the Ammoccetes 

 thyroid has been derived by a process of degeneration from that of 

 the Selachian. On the contrary, the manner in which it is eaten up 

 at transformation and absolutely disappears in its original form is, 

 like the other instances mentioned, strong evidence that we are 

 dealing here with an ancestral organ, which is confined to the larval 

 form, and disappears when the change to the higher adult condition 

 takes place. Dohrn's evidence, then, points strongly to the conclu- 

 sion that the starting-point of the thyroid gland in the vertebrate 

 series is to be found in the thyroid of Ammoccetes, which has given 

 rise, on the one hand, to the endostyle of Amphioxus and the Tuni- 

 cata, and on the other, to the thyroid gland of Petromyzon and the 

 rest of the Vertebrata. 



The evidence which I have just given of the intimate connection 

 of the two pseudo-branchial grooves with the thyroid chamber shows, 

 to my mind, clearly that Dohrn is right in supposing that morpho- 

 logically these two grooves and the thyroid must be considered 

 together. His explanation is that the whole system represents a 

 modified pair of branchial segments distinct from those belonging to 

 the Vllth and IXth nerves. The cavity of the thyroid and the 

 pseudo-branchial grooves are, therefore, according to him, the remains 

 of the gill-pouches of this fused pair of branchial segments, which no 

 longer open to the surface, and the glandular tissue of the thyroid is 

 derived from the modified gill-epithelium. This view of Dohrn's, 

 which he has urged most strongly in various papers, is, I think, 

 right in so far as the separateness of the thyroid segment is con- 

 cerned, but is not right, and is not proven, iu so far as concerns the 

 view that the thyroid gland is a modified pair of gills. 



We may distinctly, on my view, look upon the thyroid segment, 

 with its ciliated grooves and its covering plate of muco-cartilage, as 

 a distinct paired segment, homologous with the branchial segments, 

 without any necessity of deriving the thyroid gland from a pair of gills, 



