560 INTEGUMENTARY SYSTEM 



origin of the embryonic dermis in the vertebrate group is more obscure than 

 in Amphioxiis, for in the vertebrates its origin varies in different regions of 

 the developing body. Moreover, the origin of the dermal mesenchyme is not 

 the same in all species. For example, in the head region of the frog and other 

 amphibia, the dermal portion of the skin is derived in part from wandering 

 mesenchyme of the head area, at least in the anterior extremity of the head 

 and posteriorly to the otic or ear region, while immediately caudal to this 

 area the mesenchyme of the dermis is derived from the dermatomic portion 

 of the somite, together with mesenchymal contributions of the outer wall of 

 the lateral plate mesoderm. In the trunk region of the body, mesenchyme 

 from the dermatomic portion of the somite wanders off to form the embryonic 

 connective-tissue layer of the skin in the dorso-lateral region of the embryo. 

 In the middorsal region, sclerotomic mesenchyme appears to contribute to 

 the dermal area. However, the dermal layer in the latero-ventral region of 

 the body is derived from mesenchymal cells whose origin is the somatopleural 

 layer of the hypomere (lateral plate mesoderm). The dermal layer in the tail 

 arises from the mesenchyme within the developing end bud (tail bud). 



The embryonic dermis in the head region of the chick arises from mesen- 

 chyme in the head and pharyngeal areas. In the cervico-truncal region, the 

 dermatome of the somite contributes mesenchyme to the forming dermis on 

 the dorso-lateral portion of the body wall (Engert, '00; Williams, '10; fig. 

 269C), whereas latero-ventrally the mesenchyme of the future dermis springs 

 from the lateral wall of the hypomere. That portion of the developing dermis 

 overlying the neural tube appears to receive contributions from the sclerotomic 

 mesenchyme. The mesenchyme which forms the dermal layer of the skin in 

 the tail descends from the mesoderm of the end bud (tail bud). 



In the shark embryo, the origin of the embryonic dermis is similar to that 

 of the amphibia. In the mammalian embryo, a small portion of the dermal 

 tissue may arise from the dermatome; however, the greater part arises in the 

 head and pharyngeal area from the mesenchyme within these areas, in the mid- 

 dorsal region of the trunk from sclerotomic mesenchyme, and in the latero- 

 ventral region of the trunk from the outer wall of the lateral plate. In the 

 tail region, the tissue of the dermis derives from tail-bud mesoderm. Bardeen 

 ('00) concluded that the dermatome in pig and man gives origin to muscle 

 tissue. However, Williams ('10) doubted this conclusion. The fact remains 

 that the exact fate of the dermatome or cutis plate of the somite in mam- 

 mals, and even in the lower vertebrates, is not clear. 



3) Origin of Chromatophores. Chromatophores or pigment-bearing cells 

 occur in relation to the epidermis and the dermis. Dermal chromatophores are 

 numerous in vertebrates from man down to the fishes. Pigment also appears 

 in the epidermal cells, hair, feathers, and certain epidermal scales. This pig- 

 ment is derived from melanoblasts or chromatophores which lie in the basal 

 area of the epidermis or in the zone between the epidermis and the dermis 



