Endocrine Mechanisms 769 



the tritocerebral commissure of Crago. Among the vertebrates the medulla 

 of the adrenal and the posterior lobe of the pituitary are considered to be 

 derived embryologically from nervous tissue. The latter is believed to be ho- 

 mologous with the neural gland of tunicates, which, however, in addition to 

 possessing oxytocic, pressor, and melanophore activating agents, appears also 

 to have some gonadotropic activity. 



In brief, it would seem that there has been an evolution of certain en- 

 docrine tissues from tissues possessing a simple nervous or conductile func- 

 tion and located within the central nervous system, through a condition 

 where the cells exhibited secretary function while still retaining the func- 

 tional properties of nerve cells, to a condition in which the cells have become 

 specialized for endocrine activity alone. In the last instances the cells have 

 commonly become separated from the nervous system to form definite glan- 

 dular tissues or organs. In such organisms as the arthropods, which possess 

 an open circulatory system, no better access to a rich hemolymph supply 

 for such organs is to be found in the organism than the surfaces of the 

 central nervous organs which are bathed directly by the fluid contents of 

 a large blood sinus. 



Other endocrine organs have developed ontogenetically and phylogenet- 

 ically independently of the nervous system. 



It is interesting to note that, in the vertebrates, those hormones from 

 sources of ectodermal or endodermal origin are proteins (pituitary principles, 

 thyroglobulin, insulin) or at least contain nitrogen (adrenin); hormones 

 arising from tissues of mesodermal origin are characteristically steroids (sex 

 hormones, cortical principles). 



There has been some speculation in the literature as to which is prob- 

 ably the primitive integrating mechanism within animals— nervous or en- 

 docrine. Obviously both mechanisms, when broadly interpreted, extend to 

 all forms of living organisms, both unicellular and multicellular. The phe- 

 nomena of excitation and conduction, the basic underlying activities in the 

 physiology of the nervous system, are common to all cells, as are also the 

 intracellular influences of numerous chemical products of activity of every 

 cell. As was pointed out in the introduction to this chapter, some organizing 

 and diff^erentiating forces obviously had to precede both of these coordinating 

 mechanisms in both phylogeny and ontogeny. In development gene-induced 

 diff^erentiation precedes organizer activity. There is no good reason to postu- 

 late that either the excitatory or the chemical coordinating factor is phylo- 

 genetically the more primitive. Both types of coordinatory mechanisms prob- 

 ably evolved simultaneously and entirely parallelly for the most part; and, 

 in response to the functional needs of larger size, and later the multicellular 

 character, of the organisms, highly specialized nervous and endocrine sys- 

 tems differentiated. 



REFERENCES 



1. Abramowitz, A. A., Physiol. Zool. 11:299-311(1938). Similarity of crustacean 

 sinus gland hormone to intermedin. 



2. Abramowitz, a. a., Hisaw, F. L., Boettiger, E., and Papandrea, D. N., Biol. 

 Bull. 78:189-201 (1940). Hypophysis and pancreatic diabetes in the dogfish. 



3. Abramowitz, A. A., Hisaw, F. L., and Papandrea, D. N., Biol. Bull. 86:1-5 

 (1944). Diabetogenic factor in crustacean sinus glands. 



