THE DEVELOPMENT OF THE UROGENITAL SYSTEM. 391 



the glomerulus to the duct and composed of smaller cuboidal cells (Fig. 345). 

 The primary mesonephric tubules are arranged segmentally, one appearing 

 in each segment as far back as the pelvic region. Thus the intermediate cell 

 mass may be considered as a series of nephrotomes, corresponding to the 

 sclerotomes and myotomes. The segmental character is soon lost, however, 

 owing to the inequality of growth between the mesonephros and the other seg- 

 mental structures, and to the development of the secondary and tertiary tubules. 

 As stated above, the first mesonephric tubules appear immediately caudal to 



Mid-brain 



^_. ,. __ Fore-brain 



Hind-brain 



Branchial groove I 



Heart Jf- 



Intestine 



Mesonephros 



^^^^^^H :! Genital ridge 



Ccelom I1 



'* Jf/- Body wall 



Lower limb bud fPB Genital eminence 



Tail 



FIG. 346. Human embryo of 5 weeks. The ventral body wall has been removed 

 to disclose the mesonephroi. Kollmann. 



the pronephros. From this point their formation gradually progresses in a 

 caudal direction as far as the pelvic region. By the further development of the 

 primary and by the addition of the secondary and tertiary tubules and the 

 glomeruli, the mesonephros as a whole increases in size and forms a large 

 structure which projects into the ccelom on each side of the body, forming the 

 so-called mesonephric or Wolffian ridge. It reaches the height of its develop- 

 ment in the human embryo about the fifth or sixth week, at which time it ex- 

 tends from the region of the heart to the pelvic region (Fig. 346). Each organ 



