IN RATS AND GUINEA-PIGS. 29 



are black at birth remain black; those which are yellow remain yellow; 

 and those which contain at birth black and yellow hairs interspersed remain 

 in that condition ever afterward. This indicates that every portion of the 

 epidermis has its pigment-forming capacity early and finally differentiated. 

 In spotted animals the capacity to form hair and skin pigments is trans- 

 mitted only to certain portions of the epidermis. Our statistical studies 

 make it clear that guinea-pigs and rats (and probably other spotted mam- 

 mals also) transmit with a good deal of constancy definite amounts of 

 pigmented surface, but that, in guinea-pigs at least, the distribution of this 

 pigmentation over the body is not strictly localized in the germ. A certain 

 amount of pigment, apparently, is handed over to the epidermis, but it 

 seems to be to some extent a matter of chance upon what part of the body 

 this pigmented epidermis finally comes to lie. It is not, however, entirely a 

 matter of chance. It is almost certain that the pigment will lie chiefly on 

 the dorsal surface, and if the pigmentation is not extensive, it will be re- 

 stricted to one or more of the regions which we have designated eye, ear, 

 shoulder, side, and rump areas, all of which are paired and frequently separated 

 one from another by intermediate unpigmented areas. In other cases, 

 even when adjacent areas are confluent, they show their essential distinct- 

 ness by sharp differences in color. Besides the paired areas mentioned, 

 there is an unpaired area at either end of the body. The anterior one we 

 have designated nose spot, the posterior one might be called a tail spot, 

 though in the guinea-pig it is scarcely distinguishable from the rump spots, 

 because there is no external tail. The distinct pigment spots are derivatives, 

 doubtless, of individual blastomeres set apart early in development for the 

 production of the epidermis. We know that in birds and mammals the 

 epidermis is first differentiated along either side of the primitive streak. 

 The ectoderm along the middle of the primitive streak sinks down to form 

 the neural canal, then the divided right and left halves of the epidermis 

 come together above it, while anterior and posterior to the neural invagina- 

 tion the right and left halves of the epidermis have been from the beginning 

 continuous. Very likely the nose- and tail-spot regions correspond with 

 these regions of original continuity of the right and left halves of the epider- 

 mis, while the paired areas are formed out of the epidermis lateral to the 

 neural invagination. The epidermis of the ventral side of a bird or mam- 

 mal is developed, we know, later than the dorsal portions, and in spotted 

 individuals, apparently, the amount of pigment inherited is insufficient 

 ordinarily to extend out over the blastoderm into this region. Accordingly 

 the pigment is restricted to the portions of the epidermis first differentiated, 

 i. e. } adjacent to the primitive streak. In most cases, it is insufficient to 

 cover more than portions even of that. 



