The sex of the embryo is determined at fertilisation, when the sperm contributes either a Y or X chromosome to the egg. Primordial germ cells can be detected at a very early stage. See the relevant page (under related topics) for further details.
During this stage the intermediate mesoderm becomes distinct between the paraxial somites and the lateral mesoderm which differentiates into splanchnopleure and somatopleure. The nephrogenic tissue develops.
From the solid nephrogenic cord (nephrogenic tissue), which generally originates at the 8th somitic level, the nephric duct or mesonephric duct (primary excretory duct/Wolffian duct) develops and breaks away from the cord usually at about the 9th somite. The elongated mesenchymal swellings on either side of the vertebral column are now termed mesonephric ridges and within these ridges, mesonephric vesicles develop which are attached to the mesonephric duct dorsally by a principal tubule. Torrey T.W., (1954)
According to Torrey T.W., (1954) the mesonephric duct no longer differentiates in situ from the nephrogenic cord, rather, from about the 13th somite down it grows by ‘independent terminal growth’. It reaches the cloaca in embryos of 26 to 28 somitic pairs, soon becoming attached, and the mesonephric duct develops a lumen.
Very few primordial germ cells at this stage are left in the hindgut endoderm, most are migrating through the dorsal mesentery and along the peritoneum of the inner surfaces of the mesonephric folds where primitive gonadal folds are beginning to develop. Witschi E., (1948) The gonadal ridges appear as mesodermal proliferation’s along the medial surface of the mesonephros. O’Rahilly R., Müller F., (1987) p168
The coelomic epithelium of the gonadal ridge proliferates and invades the underlying mesonephic stoma. Short cords may be observed. Gillman J., (1948) An evagination of coelomic epithelium overlying the mesonephros in the thoracic region is thought to be the primordium of the paramesonephric (Müllerian) ducts. Faulconer R.L., (1951). According to O’Rahilly R., Muecke E.C., (1972), other reports suggest that the ducts develop during stages 17 and 18.
The cloacal membrane rotates, O’Rahilly R., Müller F., (1985), and a mesenchymal condensation appears in the midline, the genital tubercle which is indicated in a reconstruction of Carnegie Embryo No. 792. O’Rahilly R., Müller F., (1985) Fig 10 and also in Gasser R.F., (1975) p111. According to Spaulding M.H., (1921) a shallow depression extends the length of the tubercle, known as the urethral groove and the cloacal membrane sits at the base of this groove. Although the internal separation of the cloaca by the urorectal septum is not yet complete, a transverse bar of mesenchyme apparently spans the urethral groove forming an anal pit at the basal end of the tubercle.
The mammary crest may be identified.
The urogenital sinus consists of the a definitive sinus part, and a vesico-urethal part, into which the mesonephric ducts and ureters open according to O’Rahilly R., Müller F., (1987)p 215
The genital tubercle has formed a ‘compressed ,conical protuberance’ and the future glans and shaft regions of the phallus may be determined. Spaulding M.H., (1921). The urethral folds merge distally into the glans region of the tubercle and basally with the anal tubercles. Nipples appear on the mammary crest.
The gonads begin to differentiate. They become a distinct swelling extending to the rostral pole of the mesonephros and primary sex cords develop in both the cortical and medullary regions of the gonad. According to Larsen W.J., (1993), the paramesonephric ducts are complete and the presence of testis determining factor (TDF) in embryos containing the Y chromosome (male) induce the differentiation of cells in the medullary region of the primitive sex cords to Sertoli cells. Cells in the cortical region degenerate. In the absence of TDF, (female), the primitive sex cord cells degenerate and the mesothelium of the gonadal ridge produces secondary cortical sex cords which later begin to differentiate into follicle cells.
By this stage the genital tubercle has elongated and according to Spaulding M.H., (1921), is now termed a ‘phallus’ because labioscrotal swellings have appeared lateral to the genital tubercle in some embryos of this stage. This is based on studies of embryo’s between 16 and 17mm long although some may belong to stage 19.
By this stage the mesonephric duct and ureter open almost independently into the future bladder O’Rahilly R., Müller F., (1987)p 231
The development of the gonads now reaches a ‘phase of differentiation’ when they become histologically distinct in the male and female. The presence of an ovary is generally determined by the absence of testis cords rather than development of a female specific structure. O’Rahilly R., (1983a) It has been suggested that germ cells become preferentially associated with the dominant tissue of the gonad. i.e. the cortex in the female and medulla in the male. Zuckerman S., Baker T.G., (1977) According to Larsen W.J., (1993), in the male gonad, the differentiating Sertoli cells are now organising themselves into testis cords and the future testis begins to round up, reducing its contact area with the mesonephros. A layer of connective tissue, the future tunica albuginea begins to develop and will separate the degenerating cortical sex cords from the coelomic epithelium.
The urogenital opening, when the membrane ruptures, extends from the base of the phallus to the glans area. Spaulding M.H., (1921). The anal membrane and the perineum become defined
The paramesonephric ducts (Müllerian) approach the caudal pole of the mesonephros.
According to Spaulding M.H., (1921), in embryo’s of 21-23mm, it is possible to tell the sex of an embryo by the length of its urogenital opening. The urethral folds are pronounced, bordering the urogenital opening which extends from the base of the phallus to the distal, glans area. The labioscrotal swellings are enlarged, separated from each other by a triangular prolongation of the base of the phallus and from the phallus itself by grooves.