DEVELOPMENT OF THE GENITO-URINARY SYSTEM. 927 



The order of eruption of the permanent teeth is as follows : 



The two central incisors of the lower jaw appear from the sixth to the eighth year. 



The two central incisors of the upper jaw appear from the seventh to the eighth year. 



The four lateral incisors appear from the eighth to the ninth year. 



The four first bicuspids appear from the ninth to the tenth year. 



The four canines appear from the tenth to the eleventh year. 



The four second bicuspids appear from the twelfth to the thirteenth year. 



The above are the permanent teeth which replace the temporary teeth, The per- 

 manent teeth which are developed de novo appear as follows : 



The first molars appear from the sixth to the seventh year. 



The second molars appear from the twelfth to the thirteenth year. 



The third molars appear from the seventeenth to the twenty-first year. 



Development of the Genito - Urinary System. 



The genital and the urinary organs are developed together and are both preceded by 

 the appearance of two large, symmetrical structures, known as the Wolffian bodies, or 

 the bodies of Oken. These are sometimes called the false or the primordial kidneys. 

 They appear at about the thirtieth day, develop very rapidly on either side of the spinal 

 column, and are so large as to almost fill the cavity of the abdomen. Fig. 307, rep- 

 resenting a specimen in the possession of Prof. Dalton, shows how large these bodies 

 are in the early life of the embryon, at which time their function is undoubtedly very 

 important. 



Very soon after the Wolffian bodies have made their appearance, we can distinguish, 

 at their inner borders, two ovoid bodies, which are finally developed into the testicles, 

 for the male, or the ovaries, for the female. At their external borders, are two ducts, 

 on either side, one of which, the internal, is called the duct of the Wolffian body. This- 

 finally disappears, in the female, but it is developed into the vas 

 deferens, in the male. The other duct, which is external to 

 the duct of the Wolffian body, disappears, in the male, but it 

 becomes the Fallopian tube, in the female. This is known as 

 the duct of Muller. Behind the Wolffian bodies, are devel- 

 oped the kidneys and the suprarenal capsules. 



As the development of the Wolffian bodies attains its maxi- 

 mum, their structure becomes somewhat complex. From their 

 proper ducts, which are applied directly to their outer bor- 

 ders, tubes make their appearance at right angles to the 

 ducts, which extend into the substance of the bodies and be- FIG. sot. Fatal pig, % of an 

 come somewhat convoluted at their extremities. These tubes ?n the possession f^roTSa" 

 communicate directly with the ducts, and the ducts them- * . 2 anterior extremity . 



selves open into the lower part of the intestinal canal, oppo- ' 3, posterior extremity; 

 site to the point of its communication with the allantois. The inai wails have' been cut away" 

 tubes of the Wolffian bodies are simple, terminating in single, SthfwoVtnbo^s 1 ' 08 ^ 011 

 somewhat dilated, blind extremities, are lined with epithe- 



lium, and are penetrated, at their extremities, by blood-vessels, which form coils or con- 

 volutions in their interior. These are undoubtedly organs of depuration for the embryon 

 and take on the function to be subsequently assumed by the kidneys ; but, in the female, 

 they are temporary structures, disappearing as development advances, and having noth- 

 ing to do with the development of the true urinary organs. 



The testicles or ovaries are developed at the internal and anterior surface of the Wolf- 

 fian bodies, first appearing in the form of small, ovoid masses. Beginning just above 

 and passing along the external borders of the Wolffian bodies, are the tubes called the 

 ducts of Muller. These at first open into the intestine, near the point of entrance of the 



