288 THE PROTOCHORDATA. 



buccal cavity, or branchial sac, have been described above. 

 The most serious objection which has been raised against 

 the comparison of the hypophysis of the Ascidians with 

 that of the craniate Vertebrates is, that in the former 

 the hypophysis opens, not at an ectodermal surface into 

 the stomodoeum, but at an endodermal surface (behind the 

 stomodoeum) into the branchial sac. This is undoubtedly 

 the case in some Ascidians, e.g. Distaplia, and probably 

 also in Clavelina, etc. In Ciona, however, as I can state 

 after renewed study of the question, it apparently opens at 

 first into the buccal cavity precisely in the line of junction 

 between the stomodoeum and the branchial sac, so that its 

 upper margin is continuous with the stomodoeal epithelium, 

 while its lower margin is continuous with the epithelium 

 of the branchial sac. 



It is probable that too much stress has been laid on the 

 question whether the hypophysis of the Ascidians opens 

 at an endodermic or at an ectodermic surface, and that 

 thus the attention has been diverted from the essential 

 fact that the hypophysis opens into the buccal tube at the 

 entrance to the branchial sac. In the case of the Ascid- 

 ians, therefore, I should also regard the fusion of the 

 hypophysis, whether with the ectoderm of the stomodoeum 

 or with the endoderm of the branchial sac, as being in 

 itself non-essential, while the actual opening of the hy- 

 pophysis (itself derived by constriction from the nerve- 

 tube) into the buccal cavity, apart from the question of an 

 ectodermal or endodermal surface, is the essential point. 



