1. About 19 per cent. of the dry weight of the normal pig testicle is fatty matter. Histologically this fat occurs largely in the cells of the seminal tubules, and especially in the Sertoli cells.
2. During spermatogenesis the fat of the Sertoli cell passes inward for the nutrition of the spermatids and spermatozoa. During this passage its character is altered from a neutral fat to a lipoid.
3. About 30 per cent. of the dry weight of the cryptorchid pig testicle is fatty material. Histologically this fat lies within the seminal tubules, partially filling the Sertoli cells. The spermatogenic cells have completely disappeared.
4. We conclude that the presence of such an excessive amount of fat in the cryptorchid testicle is due to the absence of the spermatogenic cells which normally utilize during their development the fat furnished by the Sertoli cells.