1. Three thymus ‘buds’ can be originally distinguished in trout, in the thickened epithelium above the 1st, 2nd, and 3rd gill-arches, at the junction of ectoderm and endoderm.
2. Differentiation spreads almost immediately into the ectoderm between the original buds, and a continuous differentiating area is formed which is partly ectodermal and partly endodermal.
3. Repeated division of the epithelial cells, beginning a few days before hatching, gives rise to the small thymus cells. The latter are not derived from immigrating lymphocytes, as has been stated by Hammar and Maximow.
4. Emigration of thymus cells may take place at all ages.
5. Immigration of connective tissue and vascular elements from the mesoderm begins just before hatching, but such immigration is not active till a week later.
6. The large elements in the adult thymus are mesodermal, with the exception of the epithelial cells at the edges, and a few mucus gland-cells, similar to those occurring in the ordinary epithelium.
7. In both male and female trout the thymus undergoes almost complete involution, when the fish is between 2 and 2½ years old. There is no evidence that the time of involution is connected with the time of sexual maturity.
The work was done in the Department of Zoology and Comparative Anatomy, University Museum, Oxford. My thanks are due to Professor E. S. Goodrich, F.R.S., for reading this paper and making suggestions, and for laboratory facilities, and also to Mr. D. F. Leney of the Surrey Trout Farm for supplying and fixing material.
A grant towards the expenses of the work was made by the Christopher Welch Trustees.
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