DEVELOPMENT OF THE UKO-GEN1TAL OEGANS. 



1329 



fusing posteriorly, give rise to the scrotum in the male. In the female the slit-like 

 opening of the uro-genital canal retains its position, and its margins becoming elongated, 

 form the labia minora. The vaginal opening arises, as we have seen, by a downgrowth 

 of the fused Miillerian ducts which lies in front of the rectum and behind the primitive 

 uro-genital canal. The latter becomes the urethra in the female. In the male the slit- 

 like opening of the uro-genital canal is prolonged anteriorly by an active growth at the 

 base of the genital eminence, and its margins uniting, give rise to the penile portion of 

 the urethra. 



Notochord 



^Muscle plate 



Mesentery 



Wolffian duct 



Mesonephric 

 'tubule 



Umbilical vein 



Alimentary canal 



Body cavity 



THE WOLFFIAN DUCT AND EMBRYONIC SECRETORY ORGAN. 



The Wblffian Duct. The Wolffian duct arises in the mesoderm, about the 

 fifteenth day, as a solid cord of cells occupying a position immediately to the lateral side 

 of the protovertebral somites and to the medial side of the body cavity (Fig. 1038). When 

 first recognised the duct lies immediately beneath the ectoderm, and as it grows backwards 

 to reach the cloaca it is often 



; found to be intimately con- v^S^v Neural tube 



nected with the ectoderm. 

 This close connexion of the 

 duct with the ectoderm, in 

 the early stages, is by some 

 authorities supposed to in- 

 dicate a primitive ectodermal 

 origin of the canal, but by 

 others, and apparently with 

 more reason, to be a trace 



I of the opening of ducts on 

 the surface of the body, such 

 as exists in connexion with 

 the excretory organs of 

 lower animals. During 

 the third week the cellular 

 cord which represents the 

 Wolffian duct acquires a 

 lumen, and about the end 

 of the same week the duct FlG - 1038. TRANSVERSE SECTION THROUGH THE TRUNK OF A HUMAN 

 in its growth reaches the EMBRYO OF ABOUT 5 MM ' LENGTH ' 



cloaca. As soon as the 



cloaca has become divided into dorsal and ventral subdivisions, the Wolffian duct is 

 seen to end in the caudal part of the ventral subdivision, which becomes the bladder 

 and uro-genital canal (Fig. 1040). 



The mesonephros or Wolffian body is developed in the mesoderm of the " intermediate 

 cell mass," immediately adjoining the Wolffian duct, and consists of a number of trans- 

 versely arranged canals or tubules, each of which opens by one end into the Wolffian 

 duct, while its other extremity ends blindly. These transverse tubules, like the canal 

 into which they open, are at first solid cellular structures, and only later acquire a distinct 

 lumen. Increasing rapidly in size and number, the tubules become twisted and tortuous, 

 and the blind end of each dilates to form a capsule invaginated upon itself and containing 

 a bunch of capillary blood-vessels similar to the glomeruli of the adult kidney. It would 

 appear that primitively one tubule is developed in the portion of the intermediate cell 

 mass (nephrotome) corresponding to each mesodermic somite, but, in higher vertebrates 



.at all events, such a correspondence between the number of somites and the number 

 <>f tubules cannot be demonstrated. In the posterior part of the mesonephros the 

 number of tubules is very numerous, and greater than the number of segments in 

 this region. The tubules of the mesonephros arise in all segments from the sixth cervical 

 to the third lumbar. The tubules in the anterior part atrophy and disappear at a very 

 early time, even while others are being formed towards the caudal end of the embryo. 

 When at its greatest development (fifth to eighth week) the mesonephros forms a 

 relatively large glandular mass, composed of tubules resembling in a general way 

 those of the adult kidney, which projects into the dorsal part of the body cavity, and 

 extends from the region of the liver to the caudal end of the body cavity. Along its 

 lateral aspect lies the Wolffian duct. 



85 



