Degenerate and Rudimentary Organs

With the supplanting of the pronephros and mesonephros by the metanephros among amniote vertebrates, there are left behind several structures in the developing animal that are deprived of their original usefulness. Some of these structures, like the mesonephric tubules which become transformed into the epididymal tubules of the male, are rescued and made over to serve a new function. Many other structures, however, degenerate, finding their way to the anatomical rubbish pile where they remain as useless parts of the animal mechanism, even becoming a source of pathological ills.

There are various rudimentary organs also that are useless because they never attain functional efficiency. Unlike degenerate structures which have had their day, these are incipient organs that have never completely developed.

It is quite important for the pathologist to be thoroughly grounded in comparative anatomy, since it is just these degenerate anatomical relics and rudimentary structures that are most likely to prove the focal points for the formation of cysts, tumors, and other bodily abnormalities.

The young embryo presents a condition with respect to the reproductive apparatus, for example, that suggests a hermaphrodite with the rudiments of both sexes present. As development proceeds one sex becomes dominant, and the structures which characterize the other sex fade into the background as degenerate or rudimentary remains. There results a homology or equivalence in the anatomical details of the two sexes which is summarized in the case of man in Table VI, adapted from Wiedersheim.

Table of homologies of structures derived from the mesonephros and its associated ducts

The organs in this table that are in italics are functional, while the others are either degenerate useless structures or of doubtful function.

The epodphoron of the human female is an organ lying between the layers of the broad ligament of the uterus, composed of eighteen or twenty anastomosing mesonephridia that are closed at both ends (Fig. 407). In ruminants, perissodactyls, and pigs, the mesonephridia forming the epodphoron are connected with a fragment of Gaertner’s duet, which corresponds to the Wolffian duct in the male.

The paradidymis and the paroophoron in the two sexes respectively are all that remain of the posterior mesonephridia. The paradidymis lies within the spermatic cord near the globus major of the epididymis. Both the paradidymis and its homologue in the female are found only in older embryos and young children.

The urogenital organs of man before and after descent

The ductuli aberrantes are also tubules, originally nephridia, blind at one end and opening into the duct of the epididymis. There may be one, two, or several of them, although the number is usually two. They lie between the testis and the epididymis. The “inferior ductule,” which is the more constant of the two, may attain the length of two inches in man.

The appendix epididymidis (Fig. 407), which is a degenerate tip of the mesonephros, lies upon the globus major of the epididymis. Toldt found it persisting in 29 out of 105 human autopsies. A similar structure is sometimes found in the female.

The appendix testis, a small spherical sac attached to the testis (Fig. 407), represents the tip of the Mullerian duct. It has been reported as present in 90 per cent of the cases examined.

The other end of the embryonic Mullerian duct remains in the male in the form of the vagina masculina, a small sac homologous with the vagina. It is embedded in the prostate gland along with the base of the urethra and is usually distally bifid (Fig. 407), which is additional evidence that it represents the remains of coalescing oviducts.

Around the opening of the vagina masculina is a small fold of tissue, the colliculus seminalis, that marks the ends of the Mullerian ducts, and is homologous with the hymen of the female, which partially separates the vagina from the vestibule, and likewise locates the true termination of the Mullerian ducts.