30 



Scientific Proceedings, Royal Dublin Society. 



gland is probably equivalent to the hypobranchial groove in forni- 

 cates and Amphioxus. Dohrn discusses whether this theory is 

 correct or not in the next study. 



VIII. — The Thyroid in Petromyzon, Amphioxus, and the Tunicates. 1 



In Ammocoetes — the larval form of Petromyzon — the first trace 

 of the thyroid appears at the time when the most anterior bran- 

 chial diverticula of the endoderm grow out. Immediately beneath 

 the first of these diverticula an invagination is formed, which is 

 directed downwards and somewhat forwards. This invagination is 



Anterior portion of venous circulatory system in an embryo of Pristiurus (after Dohrn). 



e. = eye. 

 i. c. = internal ant. carotid, 

 sp. a. = spiracular artery. 

 sp. = spiracle, 

 hy. = hyoid arch. 



h. v. = hyoid vein. 

 <r. a. = first gill arch, 

 s. a. = second aortic arch. 

 a. a. = first aortic arch. 



the first rudiment of the thyroid. Two parallel branches of the 

 conus arteriosus first pass round this invagination and then run 

 along a mesoderm fold — the velum — which has been formed 

 between the stomodaeum and the endoderm. These arteries are 

 the most anterior branchial arteries and as such^are homologous 



1 Ibid., vol. vi. 1885, pp. 49-92. 



