The Anatomy of the Human Peritoneum and Abdominal Cavity / Considered from the Standpoint of Development and Comparative Anatomy

by George S. Huntington

The semen enters the penis at its root through a narrow opening situated close to the junction of genito-urinary sinus and cloaca.

For a short period, therefore, the human embryo and the embryos of the higher mammalia present conditions which correspond to the permanent structure of the parts in these lower vertebrates. In human embryos of 11.5 mm. cervico-coccygeal measure (32-33 days) (Fig. 28), the cloaca appears as a short sac continuous dorsad with the intestine, ventrad with the rudiment of the urinary bladder. The larger portion of the caudal gut (postanal gut) has disappeared, having been reduced to a thin epithelial strand which gradually becomes entirely absorbed. Only the proximal portion of the end-gut is used for the development of the cloaca, which, however, at first has no external opening (Fig. 28).

The tail end of the embryo becomes more extended and between it and the umbilical cord an interval appears in which the genital protuberance develops. Behind this point the ventral cloacal wall is formed by the cloacal membrane.

A considerable interval also develops between the points of entrance into the cloaca of the intestine proper and of the allantoic stalk (urinary bladder). The growth of the mesoderm pushes the intestine against the sacral vertebræ, while the stalk of the allantois with the rudimentary urinary bladder is forced against the ventral abdominal wall. These changes prepare the way for the first appearance of the genito-urinary sinus. The neck of the embryonic bladder elongates and receives the ducts of the urinary and genital glands (Fig. 29). In embryos of 14 mm. cervico-coccygeal measure (36-37 days) (Figs. 29 and 30), the genito-urinary sinus perforates the cloacal membrane on the ventral aspect of the genital protuberance, forming the uro-genital cleft. The rectum remains closed for a few days longer. The perforation is preceded by the formation of a transverse ectodermal reduplication, producing a depression called the transverse anal fissure. This depression increases in depth until a distinct anal invagination results, known as the proctodæum, which grows as a funnel-shaped fossa toward the blind termination of the endgut. In embryos of 25 mm. cervico-coccygeal measure (8½-9 weeks) the intestine still ends in a blind pouch. The anus is, therefore, independent of the end-gut in its development. It is derived from the ectoderm and its production is analogous to the formation of the oral cavity by means of the ectodermal invagination called the stomadæum.

Finally the cloaca is converted into a ventral tube from which part of the urinary bladder, the urethra and genito-urinary sinus develop, and a dorsal tube from which the rectum is derived. This double disposition of the cloaca is accomplished by gradual changes in the entoderm and mesoderm. The entoderm proliferates until a partition is formed which separates the two divisions of the cloacal tube from each other, and the mesoderm likewise increases, surrounding the newly formed entodermal tubes with tissue from which the muscles, connective tissue and blood vessels of the parts are derived ([Figs. 28] and [29]).

This partition, the septum uro-rectale, develops symmetrically on each side, appearing first as paired folds on the right and left sides called the internal perineal folds ([Figs. 28] and [29]). When these folds have reached the cloacal membrane they complete the separation of the cloaca into two adjacent canals. Each of these canals is still closed caudad by its respective portion of the cloacal membrane, now divided into an anal and uro-genital segment. These two portions of the original cloacal membrane become perforated separately, the uro-genital before the anal. Hence the external opening of the uro-genital sinus is the first to appear, to be followed by the anal perforation. The internal perineal folds are supplemented by the formation of similar external folds, ridges of mesoderm tissue which surround the anal orifice in the form of a low wall and thus deepen the anal ectodermal invagination into the fossa of the proctodæum.

Fig. 31.—Section of pelvis of human fœtus, showing atresia recti. (Esmarch.)

These developmental stages in the formation of the end-gut are of importance because they offer the explanation of the pathological conditions which result from an arrest of development and from the failure of either the uro-genital or anal opening to form in the usual manner. These malformations must date back to an early stage, and probably have their inception in disturbances occurring in the normal development between the 15th and 23d day (embryos of 3-6 mm.). Perhaps in some cases of atresia there may be a secondary obliteration of a previously formed opening. In Fig. 31 the proctodæum persists but the perforation of the anal membrane into the end-gut has not occurred. The ectoderm of the anal fossa and the intestinal entoderm remain separated by a transverse mesodermal partition. Different degrees of this malformation are observed. The layer separating the skin from the blind end of the rectum may be so thin that the meconium contained in the latter can be felt through it. On the other hand the rectum may terminate high up in a blind pouch, which is separated from the skin by a distance of several centimeters.

We may now briefly consider the genetic, histological and mechanical conditions which the above-outlined course of development imposes on the alimentary tract.