830 GENERATION. 



of the temporary teeth. The roots of the latter are absorbed, the permanent 

 teeth advance more and more toward the surface, and the crown of each tem- 

 porary tooth is finally pushed out. The number of the temporary teeth is 

 twenty, and there are thirty- two permanent teeth. Thus there are three 

 permanent teeth on either side of both jaws, which are developed de novo and 

 are not preceded by temporary structures. 



The first dental follicles usually appear in regular succession. The folli- 

 cles for the internal incisors of the lower jaw appear first, this occurring at 

 about the ninth week. All of the follicles for the temporary teeth are com- 

 pletely formed at about the eleventh or twelfth week. 



The temporary teeth appear successively, the corresponding teeth appear- 

 ing a little earlier in the lower jaw. The usual order, subject to certain ex- 

 ceptional variations, is as follows (Sappey) : 



The four central incisors appear six to eight months after birth. 



The four lateral incisors appear seven to twelve months after birth. 



The four anterior molars appear twelve to eighteen months after birth. 



The four canines appear sixteen to twenty-four months after birth. 



The four posterior molars appear twenty-four to thirty-six months after birth. 



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



The two central incisors of the lower jaw appear between the sixth and the eighth 

 years. 



The two central incisors of the upper jaw appear between the seventh and the eighth 

 years. 



The four lateral incisors appear between the eighth and the ninth years. 



The four first bicuspids appear between the ninth and the tenth years. 



The four canines appear between the tenth and the eleventh years. 



The four second bicuspids appear between the .twelfth and the thirteenth years. 



The above are the permanent teeth which replace the temporary teeth. 

 The permanent teeth which are developed de novo appear as follows : 



The first molars appear between the sixth and the seventh years. 



The second molars appear between the twelfth and the thirteenth years. 



The third molars appear between the seventeenth and the twenty-first years. 



DEVELOPMENT or THE GENITO-URINARY APPARATUS. 



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, de- 

 velop very rapidly on either side of the spinal column and are so large as to 

 almost fill the cavity of the abdomen. Fig. 311 shows how large these bodies 

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

 very important. 



Very soon after the Wolfnan bodies have made their appearance, there 

 appear 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 ex- 

 ternal borders, are two ducts on either side, one of which, the internal, is 



