1. An investigation of the seasonal cycle of the interstitium of the testes of birds, based on sixty-four individuals of varying age, has been carried out on the fulmar petrel, Fulmaris glacialis (L.).
2. The following cells (see diagram, p. 268) occur:
(i) Fibroblasts of the sheaths of tubules, blood-vessels, and of groups of Leydig cells.
(ii) Pigment cells.
(iii) Juvenile Leydig cells (generally containing lipoid droplets).
(iv) Lipoid Leydig cells.
(v) Fuchsinophil Leydig cells.
(vi) Areolar connective-tissue cells.
3. In the young bird the juvenile cell develops into the lipoid Leydig cell at a time when the testis-tubules also indicate approaching sexual maturity. In the adult, the interstitium generally consists for the most part of lipoid Leydig cells. These exhibit mitochondria when the soluble lipoids are removed by embedding in wax. At the height of spermatogenesis, and at the beginning of epithelial breakdown and tubule-regression, most of the Leydig cells have lost much of their lipoids and some of them exhibit increasing amounts of fuchsinophil substances, as described by Benoit. A fuchsinophil cell thus appears.
4. The Lipoid cell at all seasons shows a positive Schultz reaction (for cholesterol), which corresponds in intensity to the amount of sudanophil material present. It has not been possible to demonstrate cholesterol in the fuchsinophil cell.
5. At the time when the tubules are at their maximum diameter and their degeneration is under way, mitochondria are at the greatest abundance in the Leydig cell. At this period a new generation of Leydig cells arises in the interstitium. These cells quickly become meagrely sudanophil and resemble the sudanophil juvenile cells of the immature bird. They are Schultz positive. The tubules collapse; the new Leydig generation fills the empty interstitium. They exhibit profuse mitochondria, gain in lipoid content and so the interstitium is regenerated.
6. The exhausted tubules undergo a fatty metamorphosis at the time of their collapse; at the period when the interstitium has little lipoid the tubules are full of it. Beneath this fat (also Schultz positive) arises a new tubule-epithelium. The tubules as well as the interstitium are regenerated while the young of the next generation are still in the nests. An anatomical basis for an internal physiological rhythm may thus have been shown.
7. The new interstitial cells are already heavily lipoidal and the new tubule epithelium contains spermatogonia when the petrels move seaward away from the breeding cliffs in the autumn. They return to the breeding area in November and December. Thus testis regeneration, and movement both away from and back to the breeding area, occurs while the days are getting shorter.
8. From the time the birds appear on the breeding cliffs (and apparently since the autumn) there is an increase in lipoid cells along with heightened sexuality as revealed by the ripening tubule-products. These supplementary lipoid Leydig cells seem to develop from the small non-sudanophil areolar connective-tissue cells which are prominent in the adult interstitium.
9. Whilst it is not denied that the fuchsinophil cell (‘secretory cell’ of Sluiter and van Oordt) may have an endocrine function, the present results suggest that the lipoid Leydig cell is the primary secretory component of the avian testis. When the interstitium, after reproduction, passes once more to a lipoidal phase, it is not losing its secretory function as Sluiter and van Oordt infer, but is regenerating in readiness for the next season's breeding activities.