- 1The oviduct is lined by a single-layered columnar epithelium which varies somewhat in height and in its detailed characters in the different parts of the tube. In the funnel it is of the ciliated columnar variety; then it increases in height to high columnar in the upper region of the Fallopian tube, where it is composed of two types of cells, one of which (Type I.) is non-ciliated and secretory, the other (Type II.) ciliated and non-secretory. Towards the glandular region of the tube the epithelium decreases in height. In this latter region, Type I. cell is no longer secretory, although both types of cells are still recognizable, whilst a third variety of cell (Type III.) makes its appearance. These latter cells elaborate a globular secretion at a much later period in gestation. In the uterus proper the epithelium is of the low columnar form; most of the cells are ciliated, except in the case of a few secretory cells which resemble the Type III. cells of the tube.
- 2The secretory cells (Type I.) which occur throughout the upper two-thirds of the Fallopian tube except in the region of the funnel form a fine granular secretion. This is in process of formation during the growth of the Graafian follicle and its contained Ovum in the ovary and the cells are in active secretion when the ovum is shed. This secretion is regarded as forming the albumen coat of the egg.
- 3In the glandular region of the tube (forming its lower third) the formation of albumen granules ceases, the great majority of epithelial cells being ciliated and non-secretory. Intermixed with these non-secretory cells, however, are a few cells of the third variety (Type III. cells). These cells are not actively secretory until late in gestation, when they pour out a globular secretion which is regarded as being concerned with the calcification of the shell. These Type III. secretory cells are also found in the uterine epithelium, but they are not very numerous.
- 4In the glandular region of the tube the mucosa contains numerous convoluted glands (tubal glands) lined by tall columnar cells with reticular cytoplasm, the meshes of which are occupied by fluid secretion. It is suggested that this secretion is concerned with the formation of the homogeneous basal layer of the shell (layer 1). The secretion is discharged through the cell-membrane in the form of small droplets and may either pass directly into the lumen of the oviduct or it may become stored in dilatations formed by the enlarged basal ends of the glands. These dilatations are specially distinct on the glands situated in the lower part of the glandular region, though they also occur sporadically on the glands of the upper region.
- 5In the junctional region between the tubal and uterine segments of the oviduct tubal glands are still present in the mucosa, but in addition glands similar to those of the uterus make their appearance, whilst the dilatations of the basal ends of the tubal glands are more numerous. Certain of the cells lining these dilatations form a finely granular secretion, whilst those lining the uterine type of gland form a coarsely granular secretion comparable to that formed in the basal ends of the uterine glands proper. They become actively secretory sotnewhat later than the tubal glands and their dilatations. This secretion, together with that of the dilatations of the basal ends of the tubal glands, is, we suggest, concerned with the formation of the rodlet layer of the shell (layer 2, vide p. 436).
- 6The uterine glands differ from the tubal in that they are lined by a ciliated epithelium composed of two types of granule-secreting cells: of these the more numerous type possesses a pale-staining oval nucleus which lies in the lower third of the cell, its chromatin being clumped round the nuclear membrane; the other type, of much less frequent occurrence, possesses a darkly staining nucleus, shrunken and pycnotic in appearance, and situated close to the base of the cell. Both types of cell form secretory granules which appear to be similar, but the cells can be distinguished from each other throughout the entire secretory phase, The secretory granules vary in their detailed characters in different regions of the gland. In the superficial and middle parts of the glands they are finely granular and begin to be shed at a relatively early stage—e. g., Platypus VIII., with two intra-uterine eggs in a late cleavage (blastodisc) stage. This secretion is extremely plentiful and furnishes, we suggest, the nutritive fluid which is absorbed by the egg during its growth in the uterus, and which is utilized along with the ovular yolk for embryonic growth during the intrauterine and incubatory phases of the life-cycle. It is formed at a much earlier stage than that of the deeper parts of the glands, and its production ceases before these latter parts become active.
In the deeper parts of the glands, the secretory granules are larger, more numerous, and more variable in size than those produced in their upper segments. They first appear in minute vacuoles in the cytoplasm in close proximity to the nucleus, and spread from there throughout the surrounding cytoplasm to become aggregated at the apical end of the cell. During their formation the chromatin granules of the nucleus are closely adherent to the nuclear membane, but no actual nuclear extrusions were seen. The accumulation of the secretory granules causes the apical end of the cell to project into the lumen of the gland, and results in the disappearance of the cilia and their basal granules. Sooner or later the projection with its granules becomes constricted off from the main body of the cell, and so becomes free in the gland-lumen. Eventually the granules liquefy, and the fluid, diffusing out, appears in the sections as a coagulum, whilst the cytoplasmic matrix is left as a minute granule-free, light-staining mass, which presumably disintegrates and becomes added to the secretion. These more deeply situated gland-cells do not begin to shed their secretion until the egg has attained a diameter of about 9 mm. In the stage of egg DQ, which measures 16×15 mm. in diameter and so has reached its full size, the cells lining the basal ends of the glands are still laden with secretory granules, and the latter are also abundantly present in the gland-lumina. It is suggested that this secretion is utilized in the formation of the dense massive protective layer (layer 3) of the shell, which first begins to appear in eggs of about 12 mm. diameter and which is already well developed in the full-grown intra-uterine egg DQ.