810 



DEYELOPMEXT OF THE UEO-GEXITAL OEGANS. 



with the Wolffian bodies, consist in reality of two sets of tubular 

 organs, of which one corresponds to the Wolffian bodies of the embryoes 

 of the amniota, while the other tubnlar body, already referred to as 

 being of later formation and as connected with the main Wolffian duct, 

 corresponds to the permanent kidneys of the higher animals. Balfour 



Fig. 606. 



Fig. 606. — Diagram op toe 

 Primitive Uro-genital Or- 

 gans IN THE Embryo previous 

 TO Sexual Distinction. 



The parts are shown chiefly in 

 profile, but the MuUerian and 

 Wolffian ducts are seen from 

 the front. 3, ureter ; 4, urinary 

 bladder ; 5, urachus ; ot, the 

 mass of blastema from which 

 ovary or testicle is afterwards 

 formed ; W, left Wolffian body ; 

 X , part at the apex from which 

 the coni vasculosi are after- 

 wards developed ; w, w, right 

 and left Wolffian ducts ; m, on, 

 right and left Miillerian ducts 

 uniting together and with the 

 Wolffian ducts in g c, the genital 

 cord ; ug, sinus urogenitalis ; ?', 

 lower part of the intestine ; cl, 

 common opening of the intestine 

 and urogenital sinus ; cp, eleva- 

 tion which becomes clitoris or 

 penis ; Is, ridge from which the 

 labia majora or scrotum are 

 formed. 



has also ascertained (Jour, of Anat. and Physiol., yoI. x., 1875) that in 

 the selachians both the ducts are found which exist in the amniota, 

 viz., both the Wolffian and the Miillerian ducts, but that they arise in 

 a somewhat different manner from that by which they are produced in 

 birds and mammals, inasmuch as in the selachians the duct of Miiller 

 arises by the formation of a septal partition which divides the original 

 duct through a considerable part of its length into two canals : one of 

 these, the Miillerian duct, is in communication with the pleuro-perito- 

 neal cavity in front, and opens into the cloaca behind as a separate 

 tube ; the other corresponding with the Wolffian, besides being the 

 excretory duct of the primordial kidneys, becomes the vas deferens of 

 the testicle. In the selachians, therefore, the permanent kidneys con- 

 sist of two parts, of which one, the anterior, is homologous with the 

 temporary kidneys or Wolffian bodies, while the other, or posterior part, 

 corresponds with the permanent kidneys of birds and mammals. 



Balfour and Semper have made further the interesting discovery that 

 the transverse tubes of the two parts of the primordial kidney of the 

 lower animals correspond in number and position with the vertebral 

 segments of the region of the embryo in which they are situated, — a 

 fact of great interest in vertebrate morphology, and, according to the 

 authors, leading also to important views of the morphological corre- 

 spondence of the organs in question with similar organs in the anne- 



