30 INHERITANCE OF COAT-PIGMENTS AND COAT-PATTERNS 



On this interpretation, the difference between a black-eyed white and a 

 pink-eyed white (or albino) guinea-pig is this : The neural plate of the former 

 receives a pigment potentiality from the ectoderm which is invaginated to 

 form the neural canal (out of which the retina of the eye is later formed) ; 

 the albino does not receive the pigment potentiality into the neural plate, 

 simply because that potentiality is absent from the entire ectoderm. In 

 the black-eyed white guinea-pig no part of the epidermis is pigmented, 

 because all the color-bearing ectoderm has gone into the neural invagina- 

 tion and none is left outside for the epidermis. When a very little is left 

 outside, it usually is found either upon an ear, upon one side of the fore- 

 head, or about the eye (see Castle 105), facts which indicate with consider- 

 able clearness what relation the epidermis of the head region bore to the 

 anterior part of the neural invagination, and support the view that the nose 

 spot arises out of epidermis originally anterior to the neural plate. Black- 

 eyed white guinea-pigs usually produce offspring with one or more spots on 

 the body, *. e., the amount of pigment which they transmit is commonly large 

 enough to extend lateral or anterior to the neural plate and cause epidermal 

 spots as well as pigmented eyes. Thus a pair of black-eyed white guinea- 

 pigs has recently (Nov., 1906) produced one black-eyed white young and two 

 with nose spots and an ear spot each. In spotted guinea-pigs the pigment 

 is apparently always centralized about the primitive streak. It invariably 

 passes into the neural canal (if any pigment* is inherited at all), so that 

 the eyes are pigmented; usually it extends out also into the epidermis 

 bordering on the neural plate, but its limits are variable. When the 

 amount of epidermal pigment is small, it is found most often upon the head, 

 adjacent to the portion of the neural plate out of which the eyes were 

 formed. 



If we could follow through the period of cleavage the history of the 

 pigment-forming substance of the fertilized egg, we should probably find 

 that in spotted guinea-pigs this is distributed to certain blastomeres in 

 the animal hemisphere of the egg. Apparently it is to some extent a 

 matter of chance what blastomeres receive the pigment, but the anterior 

 part of the neural invagination is sure to receive pigment, and the adjacent 

 portions of the skin are more likely than any other regions to do so. In 

 nose-spot animals, pigment is evidently distributed to the most anterior 

 epidermal cells which take part in forming the dorsal surface of the animal. 

 If, as our observations indicate, the condition is to some extent hereditary, 

 the pigment potentiality must be localized in both egg and sperm in that 



*See footnote, page 28. 



