Summary

The excellent work of K. W. Dammerman (1910) on the saccus vasculosus of fishes has hitherto been the main source of our knowledge about the morphology, the histology, and the comparative anatomy of this organ, which develops as a protrusion of the infundibular wall. Dammerman's argument in favour of the sensory nature of the saccus is convincing. Not only is the wall composed of typical sensory cells and supporting cells, but the nerve and blood supplies provide additional evidence for his view. The sensory cells are characterized by a small protoplasmic protrusion into the cavity of the organ and carry about twenty small hairs, each of which ends in a little swelling. According to Dammerman the saccus is large in sea-fish, smaller in river-fish, and almost lacking in fishes of canals and ponds. Hence he suggested that its function is to estimate the pressure of the water, and therefore the depth at which the fish is swimming.

In spite of all the information in Dammerman's paper there still remain unsolved questions. For instance, successive stages in the development of the saccus of different species have not been described, and we are therefore not in a position to make valid statements as to its homology with parts of the infundibulum of other species. Moreover, we do not yet know whether there are relations between the hypophysis and the saccus vasculosus in fishes.

In this paper we deal with the development, structure, and homology of the saccus vasculosus and the possibility of a mutual influence between its tissues and those of the orohypophysis. The cytology of the saccus, including the differentiation of its cells, together with its possible function, will be treated in a second paper.