“ PROBOSCIS PORES” IN CRANIATE VERTEBRATES. 549 1 
with the hypophysis. Probably, if a careful search be made, 
the premandibular tubes will be found to occur both in birds 
and in mammals. 
Finally, it may be urged that all these openings, water- 
pores, proboscis pores, and premandibular pores are of the 
nature of coelomostomes comparable to the excretory tubules in 
the more posterior segments of the Craniates. It may also be 
pointed out that the theory here advocated gives a clue to the 
first origin and function of the hypophysis . 1 
Summary. 
An account is given of the complex histological structure 
of the epithelium lining Hatschek’s pit in Amphioxus, and of 
the development of this pit and of the preoral pit from the 
left anterior coelomic sac and an ectodermal ingrowth respec- 
tively. The preoral pit becomes the wheel-organ of the adult. 
The ciliated cells of Hatschek’s pit are of mesodermal origin, 
but the rod-bearing cells appear to come from the ectoderm. 
The evidence is strongly in favour of Bateson’s comparison of 
the opening of Hatschek’s pit with the proboscis pore of 
Balanoglossus and the water-pore of Ecliinoderms. All these 
pores were originally paired. The anterior coelomic sacs of 
Amphioxus are homologous with the premandibular somites 
of Craniates. As shown by Ostroumoff, Dohrn, and Salvi, 
these somites form tubular outgrowths opening into, or fusing 
with, the hypophysis — a connection comparable with tlm 
“ proboscis” pores of Enteropneusta, Cephalodiscus, and 
Echinodermata. The premandibular, proboscis, and water- 
pores are all of the nature of coelomostomes. It is concluded 
that the hypophysis of the Craniata is represented in Amphi- 
oxus by the wheel-organ situated in front of the true mouth, 
and that its original function was probably to drive food into 
the alimentary canal. 
1 An abstract of this paper was read at the meeting of the Linnean 
Society held on April 19th, 1917. 
