DEVELOPMENT OF THE SKIN 567 



separated from the deeper areas of the body along the dorso-lateral region 

 of the trunk by the presence of large lymph spaces. 



b. Development of the Skin in Necturus maculosus 



The newly formed, epidermal tube of a 6-mm. embryo of Necturus con- 

 sists of two layers of epidermal cells, an outer periderm and an inner stratum 

 germinativum (fig. 267D). In the ventro-lateral region of the trunk, however, 

 these two layers are flattened greatly and may become so attenuated that only 

 one layer of flattened cells is present. Unicellular glands appear in the head 

 region and represent modifications of cells of the outer ectodermal (peri- 

 dermal) layer. 



In larvae of 18 to 20 mm. in length, the epidermis is 3 to 4 cells in thickness, 

 with the outer layer considerably flattened (fig. 267E). The dermis consists 

 of a mass of mesenchymal cells, with large numbers of chromatophores lying 

 near the epidermis. Chromatophores also lie extensively within the epidermal 

 layer; some even approach the outer periphery. According to Eycleshymer 

 ('06), some of the pigment cells of the epidermis represent modified epithelial 

 cells, while others appear to invade the epidermis from the dermis. Dawson 

 ('20) believed these epidermal pigment cells to be entirely of an epidermal 

 origin in Necturus. Dushane ('43, p. 124) considered the origin of epidermal 

 pigment cells in Amphibia in general to be uncertain but suggested "that 

 these cells also come from the neural crest" via the dermal mesenchyme. 



Later changes in the developing skin consist in an increase in the number 

 of epithelial cells and in a great increase in the thickness of the dermis, with 

 the formation of bundles of connective-tissue fibers. Associated with these 

 changes, two types of multicellular alveolar glands arise as invaginations into 

 the dermis from the stratum germinativum. One type of gland is the granular 

 or poison gland, and the other is the mucous gland. The latter type is more 

 numerous (fig. 267F). Mixed glands, partly mucous and partly granular, also 

 may appear (Dawson, '20). Large club-shaped cells or unicellular glands 

 may be observed in the lower epidermal areas, while flattened cornified ele- 

 ments lie upon the outer surface of the epidermis. 



The dermis is arranged in three layers as follows: 



(a) a thin, outer, compact layer between the lower epidermal cells and 

 the dermal chromatophores, 



(b) below this outer compact layer, the intermediate spongy layer, con- 

 taining some elastic, connective-tissue fibers as well as white fibers, and 



(c) below the spongy layer, the inner compact layer. 



The chromatophores located in the outer part of the dermal layer are of 

 different kinds (see p. 591). 



