DEVELOPMExNT OF THE URINARY ORGANS. 



1935 



slightly transverse tubules within the postero-lateral body-wall that internally communicate 

 with the body-cavity or coelom, the openings being known as nephrostomata, and externally 

 join a common canal, \\\& pronephric dud, which extends caudally and empties into the dilated 

 terminal segment of the intestinal tube, the cloaca. In relation with the inner end of each 

 tubule, but projecting freely into the body-cavity, lies a group of convoluted blood-vessels, the 

 glomerulus, supplied by branches of the aorta. These three parts of the primitive excretory 



Fig. 163S. 



Neural tube, 

 Somitic cavity, isolated 



Malpighian body of mesonephros, 

 Mesonephric tubule 



Mesonephric duct 



Parietal peritoneu: 



Body-cavity' 



Nephrostome^ 

 Sexual gland' 



Suprarenal body 



Gut-tube 



Integument 



— Cavity of somite continuous with ccelom 



Notochord 

 Nephrostome 



Pronephric tubule 



Pronephric duct 

 Mesoblast of body-wall 



Body-cavity (ccelom) 

 Glomerulus of pronephros 

 Aorta 

 Visceral peritoneum 



Diagram showing fundamental relations of pronephros (on right side) and of mesonephros 

 or Wolffian body (on left side of figure). {Wiedersheim.) 



Fig. 1639. 



organ provide for the essential requirements of the most elaborate urinary apparatus, — the pro- 

 duction of the watery constituents, the excretion of the waste products, and the conveyance of the 

 excretion so elaborated. The pronephros is fundamentally a segmental organ, the tubules being 

 so arranged that each corresponds to a single body-segment or metamere, although by no means 

 every such division contains a tubule. It may be assumed that the tubules of the pronephros 

 represent the segmental ducts which in ancestral forms extended from the body-cavity directly 

 onto the external surface of the body and thus carried of? the fluids accumulated within the 

 coelom. In consequence of the closure of this direct communication with the exterior, which 

 may be accepted as having occurred during the evolution of a more elaborate excretory system, 

 the necessity for a new path of exit is met by the formation of the common pronephric duct into 

 which the tubules open, and which, by its prolongation to and termination in the end-gut, insures 

 the escape of the excretions. 



The development of the pronephros is closely associated with the mesoblastic somites. A 

 transverse section of an early mammalian embryo (Fig. 1636) shows the paraxial mesoblast, be- 

 tween the neural canal and the cleavage of the lateral 

 mesoblast into the somatic and visceral plates, to comprise 

 two parts, the mesial forming the somite and the lateral the 

 intermediate cell-mass. It may be assumed that in the 

 higher types the early somite and the intermediate cell- 

 mass have arisen by fusion of the primarily distinct dorsal 

 and ventral mesoblastic plates (Fig. 1638). The inter- 

 mediate cell-mass soon separates into a small duci-anlage, 

 situated dorsally and in close relation with the ectoblast, 

 and a larger ventral tract comprising the remainder of the 

 intermediate cell-mass. Within this ventral area the tu- 

 bules shortly appear, and later the glomeruli. Although 

 reaching a comparatively high development in certain fishes 

 and amphibians (especially in Ichthyophis described by Se- 

 mon), in mammals the pronephros consists of a few tubules 

 connected with the duct, and even as an organ of embryonic 

 life never attains more than a feeble and transient exist- 

 ence. In the human embryo of 3 mm. length, studied by 

 Janosik, it was represented by two rudimentary tubules that 

 extended from the mesothelial lining of the body-cavity 

 towards the pronephric duct, with which one of the tubules 

 still communicated. The pronephros of the amniotic ver- 

 tebrates, therefore, must be regarded as a rudimentary 

 inherited organ which appears in response to transmitted 

 ancestral tendencies. 



The Mesonephros or \A^olffian Body. — This organ 

 may conveniently be regarded as comprising a later generation of excretorv tubules opening into 

 a common canal, the Wolffian duct, which is usually looked upon as the continuation and mor- 

 phological persistence of the pronephric duct. In their development these tubules and duct bear 

 a similar relation to the intermediate cell-mass as do those of the pronephros, only the body- 

 segments involved lie farther tailward and the strict segmental arrangement of the tubules is lost 

 owing to their multiplication and, as in mammals, precocious development. In contrast to the 



Wolffian 

 tubules 



Wolffian 

 duct 



cavity 



Longitudinal section of young embryo, 

 showing early stage of Wolffian body ; tu- 

 bules are joining duct. X 50. 



