1208 



EMBRYOLOGY. 



into the mesoblast and extends as 



or pronephros (Lankester), and is a very rudimentary organ which speedily disap- 

 pears. Behind this body and to the inner side of the Wolffian duct, between it 

 and the body-cavity, a number of tubes are formed, which communicate by one 

 extremity with the Wolffian duct, and, passing transversely toward the body-cavity, 

 terminate in caecal extremities. These tubes are called segmental tubes, and the 

 whole mass is known as the mid-kidney, Wolffian body, or mesonephros (Lankester) 

 (Fig. 775). After a time the csecal extremities become dilated and enclose a tuft 

 or glomerulus of capillary blood-vessels. As soon as the permanent kidneys are 

 formed, the Wolffian body for the most part disappears. In the male, however, the 

 vasa efferentia and rete testis of the testicle are formed as outgrowths from it. In 

 the female traces of it are left as the parovarium and epoophoron. In the male 

 the Wolffian duct becomes the epididymis and vas deferens ; in the female it under- 

 goes atrophy, and is represented only by the functionless duct of Gartner. 



Finally, in that portion of the intermediate cell-mass which lies behind the 

 Wolffian body, a differentiation of* cells takes place which results in the formation 

 of a number of convoluted tubes ; into this a hollow protrusion of the lower end 

 of the Wolffian duct grows up, and thus is formed the hind-kidney or metanephros 

 (Lankester). This is the permanent kidney. The uriniferous convoluted tubes 

 and Malpighian corpuscles are formed from the intermediate cell-mass, and the 

 collecting tubules and ureter from the protrusion from the posterior end of the 

 Wolffian duct. 



Shortly after the formation of the Wolffian body, a second duct becomes devel- 

 oped. It arises on the outer side of this body as a slight thickening of the cells 

 lining the pleuro-peritoneal cavity. This thickening then becomes invaginated 



a cord along the outer side of the Wolffian 

 body, to the posterior extremity of the em- 

 bryo. It speedily acquires a lumen, and is 

 then known as the Mullerian duct (Fig. 774). 

 In its passage to the posterior extremity of the 

 embryo it comes into close relation with the 

 Wolffian duct, and the two ducts on either side 

 become connected with their fellows on the 

 opposite side by their cellular substance into 

 a single cord, the genital cord (Fig. 776, G, c), 

 in which the Wolffian ducts lie side by side in 

 front, and the ducts of Miiller behind. These 

 latter tubes in the substance of the genital 

 cord become fused together, and open by a 

 single orifice into the hind-gut (cloaca). At 

 their anterior extremities the ducts of Miiller 

 open by a somewhat funnel-shaped orifice into 

 the pleuro-peritoneal cavity. In the female 

 the greater part of the Mullerian duct is de- 

 veloped into the Fallopian tube, but the pos- 

 terior fused portion of the two ducts is con- 

 verted into the uterus and vagina (Fig. 777). 

 In the male the greater part of the ducts dis- 

 appears ; the posterior fused portion is believed 

 to be represented by the sinus pocularis (uterus 

 masculinus) of the urethra. 



It has been seen that the W T olffian and 

 Mullerian ducts open into the common cloaca, 

 which is the termination of the intestinal 

 cavity, and into which the allantois also opens 

 in front (Fig. 776). As the allantois expands 

 into the urinary bladder this common cavity 

 is divided into two by a septum, to form the urogenital sinus in front and the 



FIG. 776. Diagram of the primitive uro- 

 genital organs in the embryo previous to sexual 

 distinction. The parts are shown chiefly in 

 profile, but the Mullerian and Wolffian ducts 

 nre seen from the front. 3. Ureter. 4. Urinary 

 bladder. 5. Urachus. of, The mass of blastema 

 from which ovary or testicle is afterward 

 formed: W, Left Wolffian body; w, w, Right 

 and left Wolffian ducts ; 771, m, Right and left 

 Mullerian ducts uniting together and with the 

 Wolffian ducts in gc, the genital cord ; ug, Sinus 

 urogenitalis; i, Lower part of the intestine; 

 cl, Common opening of the intestine and uro- 

 genital sinus; cp, Elevation which becomes 

 clitoris or penis ; is, Ridge from which the labia 

 majora or scrotum are formed. 



