Histological Studies On the Gonads of the Fowl
Authors: Fell Honor B. 
Published: April 2021

(1) The histological study of the gonads of the eight birds described showed them to be hens at different stages in sex reversal. No.1 had changed completely from a fertile hen to a fertile cock which was almost indistinguishable from a normal male. The left gonads of Nos. 2 and 3 were found to contain mature seminiferous tubules, and a small testis was in each case present on the right side. The ovary of No. 4 was largely occupied by typical immature tubules, that of No. 5 by atrophic immature tubules, and those of Nos. 6, 7, and 8 by sex cords and small tubules of an embryonic type.

(2) In every case the development of testicular tissue was preceded by ovarian atrophy or disease. Oöcytes were found only in the left gonad of No. 2, and then only in small numbers.

(3) The mode of formation of the spermatic tissue appeared to be almost identical with the embryonic process, as follows : 1. A thickening of the germinal epithelium was produced by the enlargement of the component cells, and sometimes by the formation of several layers ; 2. columns of cells, or sex cords, proliferated inwards from the peritoneum ; 3. these enlarged to form tubules. The possibility that other peritoneal derivations, viz., the remains of the medullary cords and the granulosa cells of atretic follicles, play some part in the formation of new germinal tissue is admitted, although no histological evidence of this was found.

(4) Enormous numbers of typical "luteal" cells were present in all the gonads, with the exception of those of No.1, and were found to be derived from undifferentiated sex cords in precisely the same way as in the embryo and young chick. The histological evidence indicated that the process of transformation of the sex cords into "luteal" cells was one of degeneration, but, on the other hand, the theory that these cells are responsible for the exhibition of female plumage is supported by the fact that in the gonads of the only cock-feathered individual (No. 1) such cells were not found.

(5) The problem of the origin of germ cells was briefly reviewed, and it was concluded that in the present instance the newly formed gonocytes were of somatic (peritoneal) origin, since the nuclei of all the cells of the germinal epithelium contained polarised loops of chromatin, often intimately twisted, which appeared to represent early meiotic figures, and which were linked by a series of intermediate stages with spermatocytes in the characteristic contraction phase of the pachytene.

(6) The degree of differentiation of the neoplastic germinal tissue was highly variable 1. in the different individuals, and 2. in the gonad of the same bird. This is probably due in the former case to metabolic conditions which have been discussed elsewhere in a paper by F. A. E. Crew, and in the latter to the morphological environment of the sex cords, the dense regions of the pre-existing ovarian stroma being a less favourable medium for growth and differentiation than the looser parts. On this latter supposition was explained the fact that in most cases of hermaphroditism in which an ovotestis is present, the spermatic tissue is described as occurring median to the ovarian portion. Probably the reason for this is that in the atrophic ovary the cortex is usually composed of dense stroma, while the hilar portions consist of loose fibrous cords which would offer but little resistance to growth.