The Development of the Monotremata.—Part V. Further Observations on the Histology and the Secretory Activities of the Oviduct prior to and during Gestation.
Authors: Hill Catherine J. 
Published: April 2014

SUMMARY.
The Fallopian tube is lined by a single layered columnar epithelium, which varies in height and in cellular character in different parts of the tube. In the oviducal funnel (infundibulum) it is of the tall columnar type, and is composed of two types of cells, non‐ciliated secretory and ciliated non‐secretory. The secretory cells form a clear fluid which coagulates under the action of fixatives to form a translucent jelly‐like material. This fluid is elaborated during the growth of the oocyte and is shed just prior to ovulation. It is very plentiful and completely fills the space between the ovary and the infundibular membrane which comes to enclose the ovary sometime before the oocytes have attained their full size.
The oviducal funnel passes over into a narrow, thicker‐walled neck region which is lined by an epithelium similar to that lining the funnel proper except that its cells are not actively secretory. It contains, however, a number of mucoid cells which closely resemble goblet cells in appearance and which lie scattered through the epithelium or are aggregated in groups of 2–3 cells at the bases of the folds. These cells form a clear mucoid‐like secretion which is shed into the lumen of the tube just before the egg leaves the follicle and during the early fertilisation stages. It is not a very plentiful secretion and probably merely serves, like the infundibular secretion, as a lubricant which facilitates the passage of the egg into the upper part of the tube.
Behind the infundibular and neck regions the Fallopian tube is lined by a tall columnar epithelium, which is concerned with the formation of the albumen of the egg. This albumen coat consists of two parts; an inner, denser, and thicker layer which forms, with the extremely thin zona, the zona‐albumen layer of Flynn and Hill, and a more fluid, much thinner layer between the latter and the shell‐membrane.
The dense albumen is formed in the non‐ciliated cells of the epithelium lining the upper two‐thirds of the tube as a finely granular material, which becomes aggregated at the apices of the cells. This secretion is very plentiful; a small quantity of it is shed before the oocyte leaves the follicle, but its main bulk is shed during the fertilisation stages, after the egg has reached the Fallopian tube.
The fluid albumen is secreted in the lower third or glandular segment of the tube, no dense albumen being formed in this region. The fluid albumen is secreted by elongated non‐ciliated cells, which shed a finely granular material into the tube‐lumen slightly later than the dense albumen (fertilisation stages) and concurrently with the secretion of the tubal glands. This secretion can be seen as a very fine flocculent material in the lumen of the tube, which can easily be distinguished from the more homogeneous coagulum formed by the secretion of the tubal glands. Compared with the dense albumen, the amount of fluid albumen produced is quite small.
The secretory activity of the upper two‐thirds of the Fallopian tube is confined to certain cells of the lining epithelium, but in the lower third or glandular region, in addition to the secretory epithelial cells, numerous convoluted glands appear in the mucosa. These glands (tubal glands) form a very plentiful fluid secretion which is in process of being shed before the ovum leaves the follicle. This secretion is produced earlier than required, and so is stored in the lumina of the glands and in their dilated basal ends, and is only later utilised for the formation and subsequent growth of the basal layer of the two‐layered shell. The egg increases in diameter from 4 mm., when this layer first appears (Echidna FA), to 16–5 × 15 mm. in the full‐grown egg (Platypus DQ). During this time the basal layer first increases in thickness, reaching a maximum of 0–0036 mm. in Echidna A.23.7.29, an egg with a diameter of 5–6 mm., and then gradually thins out until it reaches a thickness of 0–0018 mm. in the full‐grown egg. These changes in the basal layer are not simply the result of mechanical stretching, but the layer grows in surface extent as the result of the intussusception of new secretory material stored in the gland‐lumina.
In the junctional region between the tubal and uterine segments of the oviduct, tubal glands are still numerous, whilst uterine glands have made their appearance and, as the uterine segment is approached, gradually replace those of the tubal variety. Definite spherical dilations of the basal ends of both types of glands are present in this region. The uterine glands are lined by a tall columnar epithelium composed of ciliated and non‐ciliated cells. The latter are secretory and produce a fine granular secretion. The secretory cells are not numerous in the upper portions of the glands, but in their deeper parts they outnumber the non‐secretory variety. The secretion is shed just prior to the appearance of the rodlet layer of the shell (stage FC). It is stored in the dilated basal ends of the glands and is concerned, we believe, with the formation of the rodlets of the rodlet layer.
The secretion formed by the cells lining the basal dilations of the tubal and uterine glands is liberated about the same time as that of the uterine glands, and is also stored in their lumina as a fine granular material, easily distinguishable from the secretions of the tubal and uterine glands. The rodlets increase greatly in length during the growth of the egg, reaching a maximum length of 0–016–0–018 mm. and 0–019–0–021 mm. in Wilson and Hill's eggs PP & E respectively, whilst at the same time they become separated by spaces owing to the stretching of the basal layer. These spaces are filled by a finely granular matrix‐like substance. The rodlets seem to increase in length at the expense of the material produced by the uterine glands, which diminishes in quantity as the rodlets grow, whilst the material which furnishes the matrix between the rodlets is derived, we believe, from the finely granular secretion of the cells lining the dilated ends of the glands. The two‐layered shell reaches its maximum thickness about the time the egg has reached a diameter of 10 mm., by which time the store of secretions in the dilated ends of the tubal and uterine glands is almost exhausted.
The secretory cells lining the upper and middle portions of the uterine glands after the shedding of their primary or rodlet secretion proceed to elaborate a second secretion, which is finely granular and is shed during late cleavage (blastodisc) stages, e. g., that of Platypus VIII. This secretion is voluminous and furnishes, we suggest, the nutritive fluid, which is stored in the uterine lumen and absorbed by the egg during its growth.
In the deeper parts of the glands the rodlet secretion is shed somewhat later than it is in their upper portions, with the result that the formation of a second secretion, which likewise occurs here also, is somewhat delayed and only begins to appear when the nutritive secretion in the upper parts of the glands is already in process of being shed. It is coarsely granular in character, and does not begin to be shed until the egg has attained a diameter of about 9 mm., but the height of secretory activity is not reached until the egg has attained its full size (Platypus egg DQ, with a diameter of 16.5 × 15 mm.). We regard this secretion as furnishing the massive protective layer of the shell (layer 3), the laying down of which is completed only after the egg has finished its growth and reached its full size.
The rodlet secretion is elaborated throughout the entire length of each uterine gland, and is produced in greatest quantity in the junctional region and the immediately adjoining (upper) part of the uterus, and only quite sparsely, if at all, in the remainder of the uterus.
The secretions which give origin to the nutritive fluid and layer 3 of the shell have their main seat of formation in the uterine glands situated in the body of the uterus, and are produced only in small amount in those of the upper part of the same. Potentially the secretory cells of the uterine glands are capable of producing three secretions, and some may actually do so; others may form only one secretion and yet others, two.