506 



Special Vertebrate Organogenesis 



present in the body during pregnancy 

 (Davis, Boynton, Ferguson, and Rothman, 

 '45). 



There is much evidence that male hor- 

 mone affects the vascularization of certain 

 localized areas of skin. The highly vascular 

 comb of fowl has long been used as a trust- 

 worthy and measvirable indicator of the 

 presence of male hormone. The reddening of 

 the sex skin in certain primates and the 

 skin of the legs of the night heron are simi- 

 lar examples of the response of local cu- 

 taneous blood vessels to sex hormone stimu- 

 lation. Hamilton ('48) emphasizes the im- 

 portant role of the sex hormones in regulat- 

 ing the vascularization of the human skin 

 (complexion) ; and spectrophotometric analy- 

 sis has shown that the pallor of the white 

 eunuch is mainly the result of lack of hemo- 

 globin in the cutaneous blood vessels. 



Regional DiflFerences of the Skin Established 

 Early in Ontogeny. Very little is known about 

 the time of origin of regional specificity in 

 the skin ectoderm, and this point has not 

 been systematically tested. In a series of 

 studies on the development of the dorsal fin 

 in Amblystoma embryos Bodenstein ('52) 

 found some evidence of regional differentials 

 in the response of the ectoderm of young 

 tail-bud stages. If flank ectoderm, for exam- 

 ple, is transplanted to the dorsal median 

 region, no fin is produced; but if dorsal 

 median ectoderm is transplanted to the flank, 

 over the somites, a fin is produced. In the 

 absence of more pertinent data, however, it 

 can only be assvimed that at some time dur- 

 ing the latter part of embryogenesis regional 

 specificity of the skin, which is clearly de- 

 monstrable in the early larvae, becomes 

 established. Experiments with a wide va- 

 riety of larval amphibians have demon- 

 strated that skin from various regions of the 

 body — tail, back, flank, limb buds — trans- 

 planted to different locations on the body of 

 the same individual or of another individual 

 of the same or of a different species, retains 

 its individual characteristics. The transplant, 

 moreover, metamorphoses in the manner 

 typical of the region of the animal from 

 which it came, even with regard to indi- 

 vidual details of spotting pattern (Uhlen- 

 huth, '17; Weigl, '13; Cole, '22; Reis, '30; 

 and others). Recent experiments have shown 

 that the specific central reflex relations of 

 sensory nerve fibers entering heterotopically 

 located grafts of cornea (Weiss, '42) and 

 skin (Miner, '51) depend upon the sites of 

 origin of the innervated tissues and not 

 upon the origins of the sensory nerves them- 



selves. Thus, in larval newts, tactile stimula- 

 tion of cornea transplanted to the site of the 

 ear or nasal organ will elicit the typical 

 lid-closure reflex that is obtained by touch- 

 ing the cornea of a normal eye. The in- 

 trinsic capacity of the skin tissue for the 

 differentiation of specific sensory nerve end- 

 ings has been strikingly illustrated by ex- 

 changing areas of skin between the bill and 

 leg of the duck (Dijkstra, '33). The skin of 

 the duck's bill normally contains highly spe- 

 cialized sensory nerve endings, the cor- 

 puscles of Grandry and Herbst, found no- 

 where else on the body. When an area of 

 bill skin is transplanted to the leg it be- 

 comes invaded by leg nerve fibers and the 

 characteristic corpuscles appear. If leg skin 

 is grafted to the bill, however, no such 

 sensory endings develop, despite its innerva- 

 tion by normal bill nerves. 



That the capacity of the skin to respond 

 to hormones is acquired early is evidenced 

 by the fact that skin from young amphibian 

 larvae transplanted to much older larvae 

 attains adult characteristics synchronously 

 with the host, i.e., metamorphoses earlier 

 than it normally would have done if left 

 undisturbed (Uhlenhuth, '17; Weigl, '13); 

 and metamorphic changes in local areas of 

 larval anuran skin can be initiated much 

 earlier than they would normally occur by 

 implanting pellets containing thyroxin under 

 the skin (Kollros and Kaltenbach, '52). 

 During metamorphosis Woronzowa ('32) 

 found that different regions of the skin 

 of Amblystoma tadpoles clearly showed dif- 

 ferences in their threshold of response to a 

 given quantity of injected hypophyseal 

 hormone. 



In fowl the marked regional differences 

 in the morphology and pigmentation pattern 

 of the adult plumage are directly related to 

 the order of origin of the feather papillae 

 early in embryonic life. Holmes ('35) found 

 that the feather papillae composing the in- 

 dividual plumage tracts arise in a definite 

 time and space order. In a breast tract, for 

 instance, the first longitudinal row of papil- 

 lae arises parallel to the anteroposterior axis 

 of the body at a position off-center in rela- 

 tion to the prospective tract as a whole, 

 becoming the sixth row in mediolateral or- 

 der when the tract is completed. After the 

 formation of the papillae composing the pri- 

 mary row, other rows arise in sequence to 

 the right and left of the first, and always 

 definitely oriented with reference to it, 

 until nine parallel rows are formed. 



The relation of this definite time and 



