THE INNERVATION OF GLANDULAR STRUCTURES 129 



blood to the kidneys is under the control of the nervous system, 

 and the nerve fibres, which pass into the kidney, can be largely 

 traced to the blood vessels. At present we have no conclusive 

 evidence of nerve fibres to the kidney cells themselves. The 

 kidney cells secrete substances brought to them by the blood 

 and the activity of these cells is determined by the chemical 

 nature of these substances. 



Again, however, we find in the nephric organ evidence of the 

 double segmentation upon which I have laid so much stress. 

 As I have pointed out, the segmental excretory organ of the 

 annelids becomes a double set with the formation of append- 

 ages, as is seen in Peripatus, the somatic set remaining in the 

 body, and the other appendicular set forming the coxal glands. 

 These two sets of nephric organs represent respectively the 

 two segmentations, somatic and appendicular, so characteristic of 

 the higher invertebrates ; in the vertebrate the somatic nephric 

 organs form the meso- and meta-nephros while the appendicular 

 nephric organs form the pronephros. 



The evidence of embryology shows that the first formed 

 nephric organs belong to a few segments immediately succeeding 

 the branchial segments, and form the pronephros with the seg- 

 mental duct and possibly the most anterior of the mesonephric 

 organs. Subsequently, with the development of the animal, 

 more and more segments are formed always posterior to those 

 first developed, constituting more mesonephric segments and 

 finally the segments of the metanephros. The pronephros is 

 confined to the earliest segments because they represent the ap- 

 pendage bearing segments of the invertebrate ancestor ; afterwards 

 in the vertebrate stage, with the elongation of the animal, meso- 

 somatic segments were formed but no new appendages. Con- 

 sequently in these new segments we find the formation only of 

 mesonephros and metanephros, no longer of pronephros. 



