File:Pohlman1911 plate1B.jpg

Plate 1B

Model B {No. 5; Mall no. 186; 3.5 mm.) This model, although made from an embryo of the same length as the above, shows a distinctly older stage in the development of the tract. The allantois has a much wider lumen and the angle between it and the hind gut is much more acute. The Wolffian ducts have reached the cloaca, and attach but do not open into it ventrally near the upper limit of the cloacal membrane. The tail gut is much larger both in length and in thickness, and terminates in an undifferentiated cell mass formed by itself, the chorda and the neural tube (shown as a knob-like ending). The cloaca is wider dorsally than ventrally. A stage similar to the one described was found in two other embryos (nos. 7 and 8), in both of which the allantois was wide at its apparent opening into the cloaca. The possibility of some minor degree of development abnormality suggested itself but on careful study, it was decided that the first evidences of cloacal division were at hand.

Model B, other than a better developed tail gut, a slightly higher position of the Wolffian ducts and a shorter cloacal membrane agrees with Keibel's model of the 4.2 mm. embryo. Keibel's model shows this same evidence of division; a frontal septum between the allantois and hind gut slipping down to a level approaching the upper limit of the cloacal membrane. The writer has found no embryo where the cloacal membrane approaches the level of the dermal navel, and will consider this point later.

This model shows the cloaca to be a sort of conical sac receiving the wider lumened hind gut dorsally, and the narrow lumened allantois ventrally. The latter joining the cloaca at about a right angle. The cloaca is much wider dorsally and is somewhat wedged-shaped in section. Ventrally, it is limited for about one-half of its extent by the epithelial cloacal membrane which is indicated by a surface contact. The right wall of the cloaca is concaved from before backwards while the left side (shown) is correspondingly convex, and presents a well marked furrow extending from the saddle between the allantois and hind gut to about the mid area of the cloacal membrane. This furrow is interesting because while it indicates the probable line of the later division, it was found in but two embryos of the series examined. The part of the cloaca caudal to the cloacal membrane — the tail gut — is quite short and cone-shaped. The model with the exception of the furrow conforms to Keibel's model of the His embryo EB (3 mm.)

All drawings represent 100 diameters enlargement except of Model F which is 50 diameters.

Links: plate 1 | 1 A | 1 B | plate 2 | plate 2C | plate 2D | plate 3 | plate 3E | plate 3F | Pohlman 1911

Reference

Pohlman AG. The development of the cloaca in human embryos. (1911) Amer. J Anat. 12: 1-26.

Cite this page: Hill, M.A. (2024, June 1) Embryology Pohlman1911 plate1B.jpg. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/File:Pohlman1911_plate1B.jpg

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