IN RATS AND GUINEA-PIGS- 31 



part of the germ-cell which gives rise to the anterior part of the primitive 

 streak. In selecting for the nose-spot character, accordingly, we have 

 simply selected for extreme anterior localization of a reduced amount of pig- 

 mentation. Selection for localization less far forward is apparently less 

 effectual, probably because, in that part of the germ, a cell-division, which 

 would involve unequal distribution of the pigment potentiality to the 

 daughter cells, might carry the potentiality either forward or backward in 

 the embryo, whereas original extreme anterior localization could result in 

 its transportation only forward to the most anterior part of the embryo. 

 This, while a purely hypothetical explanation, is offered as a suggestion 

 to the embryologist, who some day, perhaps, will be able to identify in the 

 germ and trace through its various stages of development the substance 

 or substances on the presence of which pigment formation depends. 



If anterior localization of pigment is possible through selection, it would 

 seem that posterior localization should likewise be possible. We began at 

 one time a series of experiments to test this point, but the results given by 

 the first selected generation were so unpromising that the experiment was 

 abandoned. It may be pointed out, however, that one might expect fixa- 

 tion of such a pattern to be more difficult, because, like the Dutch-marked 

 pattern, it involves a double selection, viz, selection for the posterior local- 

 ization of pigment sought (tail spot) and simultaneously and unavoidably 

 selection for the anterior localization of pigment represented in the black 

 eyes of all spotted individuals. 



The explanation which has been offered for the distribution of pigment in 

 partial-albino guinea-pigs will apply equally well to the case of rats, with 

 this difference: In rats the body is more elongated than in the guinea-pig 

 and pigment-reduction affects at first rather the lateral than the longitudinal 

 distribution of pigment. The back-stripe first becomes narrower, then 

 becomes interrupted, and finally drops out altogether, leaving pigment 

 only on the head and usually also sparingly on the tail (see pi. i). This 

 condition corresponds with that condition of the guinea-pig in which head 

 and tail (or rump) spots only are present, a condition very common in 

 Series H, as we have seen. As the hood becomes reduced in extent, the 

 white areas first extend forward ventrally to the mouth, then in the median 

 dorsal line between the shoulders, and simultaneously a white spot appears 

 in the forehead (see pi. i, fig. 3). Further reduction than this has not yet 

 been obtained, but it is evident that the pigmented areas are becoming 

 restricted toward the sides of the head adjacent to the eyes, precisely as in 

 guinea-pigs. 



