J. F. GASKELL 85 



in evidence, but it is here most intimately connected with the ganglia 

 and trunks of the posterior nerves, an arrangement strictly comparable 

 with Onodi's^^ description of the early development of the mammalian 

 sympathetic system. The evolution in the vertebrate kingdom of 

 the two now separate systems takes the form of a steadily increasing 

 development of the nervous or sympathetic type of cell and a relative 

 diminution and concentration of the secretory type. The final con- 

 dition reached in the mammal is a widely distributed complex sym- 

 pathetic nervous system, with a complete concentration of the secretory 

 system into the medullary tissue of the suprarenal capsules. 



The emigration from the central nervous system of the cells which 

 secrete adrenalin took place at the same time as that of the nerve 

 cells of the sympathetic system; their close association and similarity 

 of nervous control is still clearly seen in animals as high in the verte- 

 brate scale as the amphibia, where chromaffin cells are incorporated 

 in every sympathetic ganglion. 



CONCLUSIONS. 



1. The sympathetic nervous system and the adjuvant adrenalin- 

 secreting system are found in their earliest form in the annelid king- 

 dom, and consist of cells situated in the central nervous system which 

 are the common ancestors of both, and which are both secretory and 

 nervous in function. 



2. These cells are developed in the annelid kingdom parallel with 

 the development of a contractile vascular system, which possesses 

 muscles comparable in physiological actions with the muscle of the 

 vertebrate heart. 



3. This vascular muscle is regulated by the processes of the common 

 ancestral cells as well as by their secretory activity. 



4. In the primitive form contractile rhythm is an intrinsic property 

 of cardiac muscle; its nerve supply regulates the rhythm, it does not 

 initiate it. The beat is therefore myogenic, not neurogenic. 



5. The contractile vascular system of annelids is mainly branchial 

 in function. The vertebrate heart has been derived from it by the 

 growing around of the lateral body folds to form a new ventral surface. 



18 Onodi, A. D., Arch, mikr. Anat., 1885-86, xxvi, 553. 



