STUDIES ON THE PROTOCHORDATA . 319 



of the central nervous system communicated directly with that 

 of the branchial sac, and said (2, p. 515), "Somit ist die 

 Flimmergrube [dorsal tubercle] der Ascidien am ehesteu rait 

 dem Geruchsorgane [olfactory pit] des Amphioxus zu ver- 

 gleichen." 



Schimkewitsch (16) has recently put forward the same 

 opinion in that he says, " Der vordere Neuroporus [of Balano- 

 glossus] entspricht der Flimmergrube der Amphioxus-Larve 

 (Hatschek) und dem Flimmerausgang der Neuraldriise der 

 Tunicaten (Julin)." 



I consider it, therefore, well established by all this more or 

 less concurrent testimony that the hypophysis of the Ascidians 

 is represented in a simplified form by the olfactory pit of Am- 

 phioxus, both structures communicating during a longer or 

 shorter period of the development with the cavity of the 

 central nervous system by means of the neuropore. But 

 while in the Ascidians the hypophysis opens into the mouth- 

 cavity, in Amphioxus it opens dorsally to the exterior, and is 

 separated from the mouth by the notochord. 



In Amphioxus the mouth has not merely been forced by the 

 forward extension of the notochord to forsake its primitive 

 dorsal position, but it has also, ipso facto, lost its primitive 

 relation to the hypophysis, by which name we may now 

 designate the olfactory pit of Amphioxus. 



The relation of the mouth to the hypophysis is a remarkably 

 close and constant one throughout the whole of the Vertebrate 

 series. There are, however, as might be expected, some ex- 

 ceptions to the general rule. One of these exceptions is the 

 well-known case of Petromyzon, where the hypophysis, as shown 

 by Dohrn (1), Scott (17), and KupflFer (9), arises approximately 

 in the normal position for the Craniata, and is then secondarily 

 carried round to the dorsal middle line by the enormous 

 development of the upper lip which grows out between the 

 hypophysial involution and the stomodseum. 



Another exception is met with in the case of Amphioxus, 

 where it is not the hypophysis which has been carried away 

 from the mouth, but the mouth which has been separated by 



VOL. 35, PART 2. NEW SER. 2 



