30 



THE STRUCTURE OF MAN 



mencing fusion between the last two of these sometimes occur. 

 Six vertebras were once observed in a boy four weeks old ; and 



a I" 



FIG. 20A. DIAGRAMMATIC KECONSTRUCTION OF THE TAIL END OF A 



HUMAN EMBRYO (length of trunk, 8 mm. ) 



ch., notochord ; n., Wolffian tubule ; ., duct of primitive kidney ; al', intestine ; U' , 

 _^ urinary bladder ; m.a., anal membrane ; md, medullary tube ; al", post-anal gut ; 

 bl", neck of allantois ; c.u., umbilical cord. (After Keibel.) 



FIG. 20s. DIAGRAMMATIC RECONSTRUCTION OP THE TAIL END OF A HUMAN EMBRYO 

 YOUNGER THAN FIG. 20A (entire length, 4 mm.). (After Keibel.) 



Lettering as above ; in addition c.c., caudal limit of the ccelom ; a.c., caudal limit of the 

 hind-limb ; i-ii, line drawn through the anterior limit of the tail. 



Leboucq has recorded the same number in an embryo 25 

 mm. long. The opposite extreme is reached where only three 



