MELANOPHORE A TYPE OF SMOOTH MUSCLE CELL 195 



surface of the iris in the eel Steinach ('92) has observed a transi- 

 tion from the stellate melanophores to the spindle shaped cells 

 of the sphincter papillae. He says (p. 515): ''Die verastigten 

 sternformige Pigmentzellen . . . gehen dicht am Pupillar- 

 rand in einen Ring von besonders dunkeln Pigmentzellen liber 

 deren plumpe Korper sich mehr der spindelform nahern." 



We may conclude then, that, as regards external and nuclear 

 form and a melanin pigment content, no sharp morphological 

 distinction can be made between melanophores and the smooth 

 muscle cells of the sphincter pupillae. 



EMBRYOLOGICAL EVIDENCE 



Smooth muscle may be formed either from the mesoderm 

 (mesenchyme), the commonest source, or from the ectoderm, 

 (Nussbaum, M. '01). 



Investigations upon the development of the dermal melano- 

 phores show a satisfactory agreement. With a single exception^ 

 they have been found to originate from mesodermal cells 

 (Wenckebach '86, Bolk '10: Ehrmann '96, Eycleshymer '06, Wei- 

 denreich '12: Kerbert '77, Todaro '78, Zenneck '94). The chief 

 controversial point in the case of the epidermal melanophores 

 has been as to whether these cells develop from the epidermis in 

 situ, or whether, as unpigmented mesenchyme cells, they wander 

 into the region of the epidermis and become pigmented second- 

 arily. In the first case their origin would be ectodermal and in 

 the second, mesodermal. My own observations are limited to 

 several species of teleosts. In the case of Fundulus, epidermal 

 melanophores do not occur, but the origin of the dermal melano- 

 phores from wandering mesenchyme cells has been clearly demon- 

 strated by Stockard ('15). Dr. Stockard has very kindly shown 

 me a number of his own preparations and unpublished drawings 

 of the developing melanophores in Fundulus embryos. His 

 studies, made in connection with the developing circulatory sys- 

 tem, have shown that in this species the yolk melanophores first 

 appear as wandering, actively amoeboid, unpigmented mesen- 



1 Borcea, '09. 



