THE DEVELOPMENT OF THE ELASMOBRANCH PITUITARY

The early stages of development of the elasmobranch pituitary resemble closely the early stages of development of the mammalian pituitary, with this difference—that there is no invagination of the wall of the cerebral vesicle in the elasmobranch to form an infundibular lobe. The pituitary of the elasmobranch is developed entirely from Rathke's pouch, i.e. from buccal epithelium. The general relation of the pituitary to the ventricles of the brain is, in elasmobranchs, similar to that obtaining in the higher orders of vertebrates, but this is accounted for by the development of a paired saccus vasculosus, each of which pours its secretion into a common infundibular canal. The wall of this so‐called infundibular canal is lined with epithelium similar in character to that lining the saccus vasculosus. Its nervous structure is lost, being replaced by connective tissue and numerous thin‐walled blood‐vessels. There is no invasion of the wall of the canal by epithelial cells, and no hyaline bodies are formed. The pituitary body of the elasmobranch is a gland, the secretion of which is poured directly into the blood‐vessels. There is no evidence of any direct secretion by the pituitary into the brain ventricles. I have to thank Mr Richard Muir for the accompanying illustrations. The expenses incurred have been assisted by a grant from the Carnegie Fund for research work.