“Proboscis pores” in Craniate Vertebrates, a Suggestion concerning the Premandibular Somites and Hypophysis.

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Abstract
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 cœlomic sac and an ectodermal ingrowth respectively. 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 Echinoderms. All these pores were originally paired. The anterior cœlomic sacs of Amphioxus are homologous with the premandibuhir 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 the "proboscis" pores of Enteropneusta, Cephalodiscus, and Echinodermata. The premandibular, proboscis, and waterpores are all of the nature of cœlomostomes. It is concluded that the hypophysis of the Craniata is represented in Amphioxus 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.

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