Skin and Its Derivatives 



513 



ferential growth within the layers of the 

 skin itself, and that divergent whorls and 

 rosettes are controlled from their centers by 

 excess growth in the outer layer of skin. In 

 lieu of experimental data, the study of the 

 development of epidermal ridge patterns of 

 anomalous hands and feet of human em- 

 bryos has led Cummins ('26) to a similar ex- 

 planation. In many respects hair arrange- 

 ment is similar to dermatoglyphic configura- 

 tions. Regions in which hairs slant uniformly 

 in one direction are comparable to open 

 fields of ridge pattern, while irregularities 

 of hair direction localized at the points of 

 junctvu'e of several different hair slants cor- 

 respond to triradii. 



The genetic investigations of Wright C49a, 

 b; '50) on the guinea pig have contributed 

 importantly towards an interpretation of the 

 relation of genes to the development of pat- 

 tern. By means of extensive breeding experi- 

 ments he has shown that three genes, R, M, 

 and St, exert major effects on hair direction 

 in the guinea pig. A schema of the general 

 physiological processes leading to local al- 

 terations in skin growth which are due to 

 these genes and their interactions has been 

 presented (Wright, '50, p. 59). Wright's 

 studies of the genetics of normal and ab- 

 normal growth patterns of the guinea pig 

 have led to the view that the specificity of 

 gene action is always a chemical specificity 

 — the production of enzymes which guide 

 metabolic processes along particular chan- 

 nels. The development of any morphological 

 pattern is, according to this view, a chain 

 of reactions in which each gene reacts only 

 in the presence of certain conditions, in 

 part environmentally relative to the cell 

 lineage in question, in part the result of the 

 action of genes previously called into action 

 (Wright, '34a, b). 



Pigmentation Patterns. Experimental stud- 

 ies in recent years have contributed much 

 towards an vmderstanding of the numerous 

 factors involved in the development of color 

 patterns, especially those in which the im- 

 portant and widely distributed melanins or 

 granular pigments are involved. As the in- 

 vestigations have broadened it has become 

 more and more evident that the principles 

 underlying pigmentary pattern formation 

 are remarkably similar among the verte- 

 brates in general. In analyzing the develop- 

 ment of specific pigmentation patterns, at- 

 tempt has been made to determine to what 

 extent the migration, differentiation, and 

 orientation of the melanin pigment-forming 

 cells (melanophores) into distinctive pat- 



terns are dependent on their intrinsic prop- 

 erties imparted to them by their genetic 

 constitution, and to what extent upon ex- 

 trinsic, environmental factors, i.e., the tis- 

 sue substrates with which they become asso- 

 ciated ultimately. 



The Pigment Cell and Pattern Formation. 

 Owing to the well-established fact that pros- 

 pective pigment cells (melanoblasts) arise 

 from a transitory embryonic structure, the 

 neural crest, and reach the tissues with 

 which they become associated later on 

 through their migratory activities, it has 

 been possible to employ a wide variety of 

 appropriate transplantation, explantation, 

 and deficiency experiments towards clarify- 

 ing their role in pattern formation. 



In fowl and other birds melanoblasts from 

 individuals of varieties exhibiting a specific 

 type of color and pattern have been intro- 

 duced into feather primordia of individuals 

 exhibiting an entirely different color and 

 pattern. Such combinations have been ac- 

 complished by means of various grafting 

 methods (see Rawles, '48). The large body 

 of results obtained has been consistent in 

 showing that melanophores retain their spe- 

 cific characteristics (shape and color of the 

 pigment granules) and react with the for- 

 eign feather germs into which they have 

 been introduced, to produce a typical donor 

 color and pattern. In other words, the genetic 

 constitution of the pigment cell governs the 

 type of response which it manifests in a 

 developing feather. Further proof that the 

 color and pattern manifested in feathers is 

 controlled by the particular assemblage of 

 genes with which the pigment cell is en- 

 dowed has come from grafting melanoblasts 

 from male and female embryos of varieties 

 of fowl showing sex-linked differences in 

 plumage pattern, such as the Barred Plym- 

 outh Rock and F^ hybrids from crosses 

 between Rhode Island Red males and Barred 

 Plymouth Rock females. Without exception, 

 melanoblasts from prospective males reacted 

 with foreign host feather germs to produce 

 a male plumage pattern; those from prospec- 

 tive females, a female plvimage pattern, re- 

 gardless of the sex of the host (Willier and 

 Rawles, '44a, b). With reference to the lack 

 of any influence from the sex hormones of 

 the host, it should be mentioned that among 

 fowl, pigment cells show a differential sen- 

 sitivity in their response to sex hormones. 

 Melanoblasts of the Barred Plymouth Rock 

 and the cross referred to above are represen- 

 tative of a type which may be classified as 

 "insensitive," their phenotypic manifestation 



