“ PROBOSCIS PORES ” IN CRANIATE VERTEBRATES. 545 
becomes transformed into the adult mouth, and the lateral 
flaps of the oral hood develop, the preoral pit is carried into 
the buccal cavity, where it flattens out and spreads to form 
the ciliated organ. 
Now in Balanoglossus the first pair of ccelomic sacs arise in 
essentially the same way, and acquire an opening to the 
exterior known as the proboscis pore. As in Atnphioxus, so 
in B. kowalevskii, the pore is formed only on the left side. 
In B. kupfferi, however, both a right and a left pore are 
present. More than thirty years ago Bateson, in his im- 
portant papers on the development of Balanoglossus, com- 
pared the opening of Hatschek’s pit in Amphioxus with the 
proboscis pore (2), and further suggested that the proboscis 
pore and gland of Balanoglossus correspond to the hypophysis 
and pituitary gland of the Craniata. A discussion of the 
latter interesting suggestion would require a detailed study of 
the structure and development of these parts in Balanoglossus 
— a subject into which we need not enter here ; but Bateson’s 
oomparison of the pores seems to be strongly supported by 
the facts mentioned above. The homology may, of course, be 
•extended to the similar pores in Cephalodiscus, and to the 
water pores of Echinoderms. 1 
Turning now to a comparison between Amphioxus and the 
Craniate Vertebrates. That the hypophysis is an ancient 
organ which must have been possessed by the ancestor of all 
Craniates is shown by its constant presence and uniform 
development. Invariably it arises as an ingrowth of ectoderm 
just in front of the mouth and just anterior to the front end 
of the archenteron. 2 From the wall of the latter are here 
1 A comparison with the Tunicata is much more difficult. We can 
hardly avoid the conclusion that the subneural gland with its ciliated 
duct and dorsal tubercle are homologous with the hypophysis ; but of 
anterior ccelomic sacs and of proboscis pores in Ascidians we know 
nothing as yet. On the other hand, it is possible that further research 
may reveal traces of these organs in some of the less modified forms. 
2 For an excellent account of the development of the hypophysis and 
a review of the literature see the recent papers of E. A. Baumgartner 
in the ‘ Journal of Morphology,’ vols. 26, 1915, and 28, 1916. 
