DEVELOPMENT OF THE GENITO-URINARY SYSTEM 835 



on either side of both jaws, which are developed de novo and are not 

 preceded by temporary structures. 



The first dental follicles appear usually in regular succession. The 

 follicles for the internal incisors of the lower jaw appear first, this 

 occurring at about the ninth week. All the follicles for the temporary 

 teeth are completely formed at about the eleventh or twelfth week. 



The temporary teeth appear successively, the corresponding teeth 

 appearing a little earlier in the lower jaw. The usual order, subject to 

 certain exceptional variations, is as follows : 



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 OF THE GENITO-URINARY SYSTEM 



The genital and the urinary organs are developed together, and both 

 are preceded by the appearance of two large symmetrical structures, 

 known as the Wolfnan bodies, or the bodies of Okeri. These are some- 

 times called the false, or primordial kidneys. They appear at about the 

 thirtieth day, develop very rapidly on either side of the spinal column 

 and are so large as almost to fill the cavity of the abdomen. Figure 241 

 shows how large these bodies are in the early life of the embryo, at 

 which time their office undoubtedly is important. 



Soon after the Wolffian bodies have made their appearance, there 

 appear at their inner borders, two ovoid bodies, which finally are devel- 



