OF THE BODY congenital malformations of the digestive tract and of the genito-urinary organs are so closely related to the history of development of the lower parts of the embryonal body that it is impossible to separate them. They are so frequently complicated with each other, as for instance in the development of the rectum and the genitourinary organs, that the assumption of common stages of em bryonal development is justified from the malformations alone.
For the better understanding of the matter a short survey of the fetal development of these groups of organs may be given, to h reference will he made as we proceed. The details have been taken from the publications of Strahl, Kaufmann, Kolbe], and Stieda.
Even the very earliest embryonal stages in the human being show completely developed investing membranes (II. Strahl). Before the appearance of the primitive vertebra! the amnion, originating from a protrusion within the ectoderm, represents a closed cystoid fold of the embryonal integument. The vitelline membrane is still upon the open ventral side, and the lower end of the body is connected with the inner surface of the chorion by a short cord called the ventral pellicle (Ilis's Bauchstiel). Au eetoilermal diverticulum, corresponding to the allan toid duct, which later develops into the cavity of the bladder, protrudes into the pellicle.
This ventral pellicle develops later into the umbilical cord and con tains the umbilical veins and arteries and the extracorporeal parts of the vitelline membrane and of the allantois. The amnion grows rapidly and reaches the chorion, to which it intimately clings. The umbilical cord grows longer, and the extracorporeal part of the ritclline mem brane becomes obliterated. The portion of the small intestine which was originally situated in the umbilical cord and communicates with it through the ductus omphalomesentericus, has already been drawn into the abdominal cavity and the placenta still contains insignificant remnants of the vitellinc membrane. (See Meekcl's diverticultun, Um bilical hernia.) The bladder is formed from the intraperitoneal portion of the allan tois, which disappears in the duct contained in the ventral pellicle or umbilical cord, as well as the portion which runs from the future fundus of the bladder to the umbilicus, the urachus or ligamentum vesico-um bilicale medium. (See Urachus, Urachus fistula.) The formation of the bladder takes place by means of a division of the cloaca, which orginally represents at the posterior part of the body the connection between the gcnito-urinary canals on the one hand and the large intestine on the other (Kaufmtm, Keibel). Into this cavity the peritoneum is inverted from the top, dividing it into a dorsal space —the rectum, and a ventral space—the bladder. Both spaces remain in connection for a long time. (See Anal anomalies, Fistula:.) Finally the septum arrives at the exterior membrane of the cloaca, whence a frontal septum grows to meet it, and this union effects the separation between rectum and the genitourinary system. Between
both systems the perineum is now established. Between the umbilicus and the anterior membrane of the cloaca the abdominal wall advances forward, separating them from each other. Arrest of this development gives rise to abdominal and vesical fissures or epispadias.
The dorsal section of the cloaca] membrane, or anal membrane, still closes the rectum exteriorly and the anal groove is meeting it from outside. The definite perineum continuing to develop, the anal mem brane is forced more and more downward, until it finally disappears. This establishes the exterior communication of the rectum. (See Atresia ani.) Previous to the separation of bladder and rectum the genital pro tuberance arises from without and in front of the eloacal membrane, the protuberance being surrounded by the genital folds formed by the sides of the eloacal membrane. Around this rudimentary structure arise the genital ridges. Toward the end of the second month the genital protuberance shows a ventral groove which in the male forms the long urethra with the aid of the genital folds, a small gap remaining at the glans penis. It is only later that the genital ridges grow together to a raphe, forming the scrotum. (See Hypospadias and Hermaphro ditism.) In the female the genital protuberance develops to form the cli toris, the genital folds forming the labia minora which encircle the clitoris and the sinus urogenitalis or vestibulum. The genital ridges persist as labia majora. From within, at both sides of the vertebral column, the mesonephros (primitive kidney or Wolffian body) is formed about the fourth week, the excretory ducts of which (the Wolffian ducts) discharge into the sinus urogenitalis.
A strip of epithelium develops from the lateral surface of the mesone phros, which later becomes Miiller's duct, and a second cell-nest at the menial side which is called the germinal epithelium. From the latter, together with the mesonephros, the testicles arc developed, the mesone phros supplying the canals to the nuclear epithelium, while the duct of the mesonephros forms the vas deferens. Muller's ducts become involuted, leaving but a few rudimentary parts (hydatids, utriculus masculinus), while the testicles, together with their peritoneal folds, are drawn from their place of origin into the scrotum through progressive growth. (See Hernia.) In the female the ovaries develop instead of testicles from the nuclear epithelium and mesonephros, settling at both sides of the uterus along the inguinal ligament. (See Ovarian hernia.) Nothing but rudiments, Gaertner's ducts, remain of the canals of the mesonephros, while MUller's ducts develop into the tubes which unite with the uterus and vagina. (See Uterus bicornis.)