DEVELOPMENT OF THE SKIN 



561 



(Dushane, '44). Experimental embryology strongly suggests that these chro- 

 matophores are derived from the neural crest cells which in turn take origin 

 from the primitive ectoderm in association with the neural tube at the time 

 of neural tube closure. From the neural crests, the mesenchymal cells, which 

 later give origin to chromatophores, migrate extensively throughout the body 

 and to the skin areas (Dushane, '43, '44; Eastlich and Wortham, '46). 



B. Development of the Skin in Various Vertebrates 



1. Fishes 

 a. Anatomical Characteristics of the Integument of Fishes 



The epidermal layer of the skin of fishes is soft, relatively thin, and com- 

 posed of stratified squamous epithelium (figs. 263E; 264E; 265). Cornifica- 

 tion of the upper layers is absent in most instances. However, in those fishes 

 which come out of the water and spend considerable time exposed to the air, 

 cornification of the surface cells occurs (Harms, '29). Unicellular mucous 

 glands are abundant, and multicellular glands also are present (fig. 264C). 

 A slimy mucous covering overlies the external surface of the epidermis. Poison 

 glands may occur in proximity to protective spines or other areas (fig. 264F). 





UNICELLULSR G L ON 



LOOD VESSEL 



Fig. 265. Development of phosphorescent organ in Porichthys notatiis. (From Greene: 

 J. Morphol., 15.) (A) Rudiment, separating from epidermis. (B) Section of ventral 

 organ of free-swimming larva. (C) Section of fully developed ventral organ. 



