THE ADRENAL .421 



of sympathetic ganglion cells. From the capsular plexuses fine fibres 

 pass into the cortex, where they form networks around the 

 groups of cortical cells. The nerve terminals of the cortex appar- 

 ently do net penetrate the groups of cells. Bundles of nerve fibres, 

 larger and more numerous than those to the cortex, pass through the 

 cortex to the medulla. Here they form unusually dense plexuses 

 of fibres, which not only surround the groups of cells, but penetrate 

 the groups and surround the individual cells. Associated with the 

 plexuses of the medulla, less commonly of the cortex, are numerous 

 conspicuous groups of sympathetic ganglion cells. 



Development.— The cortex and medulla have entirely different develop- 

 mental histories. In the lower vertebrates (fishes) the two parts of the gland 

 continue separate throughout life. In the ascending mammalian scale, the two 

 parts become more and more closely united until in mamnials they form a single 

 organ. The cortex develops from mesoderm, first appearing in embryos of 

 about five to six mm. At about the level of the cephalic third of the mesone- 

 phros the mesothelium sends outgrowths into the mesenchyme. These out- 

 growths soon lose their connection with the main mass of mesothelium and con- 

 stitute the anlage of the adrenal cortex. The medulla has an entirely inde- 

 pendent origin, being derived from ectoderm, as part of the peripheral sym- 

 pathetic nervous system. The cells of some of the sympathetic ganglia differ- 

 entiate into sympathoblasts and phceochromohlasts , which give rise to the sym- 

 pathetic cells and chromaffin cells respectively. These cells soon separate from 

 their ganglia of origin and come to lie first near, then within, the developing 

 cortex, thus forming the medulla. 



General References for Further Study 



Flint: The Blood-vessels, Angiogenesis, Organogenesis, Reticulum and 

 Histology of the Adrenal. Contributions to the Science of Medicine, Johns 

 Hopkins Press, 1900. 



Pfaundler: zur Anatomie der Nebenniere. Anzeiger Akad. Wien, 29, 1892. 



Nagel: Ueber die Entwickelung des Urogenitalsystem des Menschen. 

 Arch. f. Mik. Anat., Bd. xxxiv. 



Stohr: Lehrbuch der Histologie B. 15th Ed. 



Prenant: Traite d'Histologie. 



