1913] Johnson: Pigment Formation in Amphibian Larvae 55 



growth of the different biophores which produce differences in 

 the constitution of the determinants and consequently qualitative 

 variations in the organism. These variations may be spon- 

 taneous resulting from intra-germinal nutritive conditions and 

 therefore not affecting all the ids at once or they may be brought 

 about by extra-germinal influences affecting all the ids at the 

 same time. 



Among Mendelians the factor hypothesis plays an important 

 role. Castle (1909) names the following color factors which he 

 finds in the case of the gray rabbit : 



"Symbol C. A common color factor necessary to the production of 

 all pigment, wanting only in albinos. 



' ' B. A factor for black, some substance which acting upon C 

 produces black pigmentation. 



" Br. A factor for brown, some substance which acting upon 

 C produces a chocolate-brown pigmentation. 



" Y. A factor for yellow, some substance which acting upon 

 C produces yellow pigmentation. 



11 I. An intensity factor, which determines whether the 

 pigmentation shall be intense (as in black and in 

 yellow), dilute (as in blue and in cream), or of some 

 intermediate degree of intensity. 



" A. A pattern factor which causes the black or brown pig- 

 ments to be excluded from certain portions of the 

 individual hairs, where yellow then shows. A 

 ' ' ticked ' ' gray coat results. When this factor is 

 present the under surfaces of the rabbit (tail, belly) 

 are unpigmented (white). The symbol, A, stands for 

 agouti, this factor having first been demonstrated in 

 the "agouti" guinea-pig. (See Castle, 1907.) 



" U. A factor for uniformity of pigmentation (in distinction 

 from spotting with white, S). 



" E. A factor governing the extension of black and of brown 

 pigmentation, but not of yellow. When most re- 

 stricted in distribution the black or brown pigments 

 are found in the eye and in the skin of the extremities 

 only, but not in the hair, when more extended they 

 occur also in the hair generally." 



The various types of coloration seen in different rabbits are 

 represented by means of various arrangements of these symbols 

 in fashion resembling the formulae for the constitution of 

 molecules of organic compounds. 



