No. 571] 



PATTERN DEVELOPMENT 



401 



wholly sundered these two areas, so that a white-collared 

 mouse results. The condition shown in Fig. 22 is similar, 

 except that the separation has taken place on the right 

 side only, between the neck and the shoulder patches of 

 but one half of the body. A break between the two neck 

 patches of opposite sides is further indicated in this 

 figure by the deep median reentrant back of the ears. 



In all four diagrams the areal restriction of the 

 shoulder patches is shown, but in varying degrees. In 

 Fig. 21, the pigment has not spread to the feet, leaving 

 these white, and so in the other figures, but to a greater 

 degree. A median linear break between the shoulders 

 indicates the restriction of the patches of opposite sides 

 at this point, which in Fig. 22 is more clearly perceptible. 

 The posterior limits of the shoulder patch are further 

 shown in this diagram, by the beginnings of a break 

 between the shoulder and the side patches. In Fig. 24 

 this break is no longer interrupted, but clearly separates 

 the two areas. Further, the side patch has dropped out 

 on the left. In Fig. 23 an imperfect separation of 

 patches on the posterior part of tin 1 body lias taken place. 

 On the right-hand side the shoulder patch, which in mice 

 is of considerable extent, has broadly separated from the 

 side patch, while on the left-hand side a long transverse 

 break has taken place between the side and the rump 

 patches, with two island-like white spots between, the 

 anterior of which probably marks the transverse line of 

 stress between shoulder and side patches, the posterior 

 the median line of breaking between the two side patches. 

 A slight indentation in the pigmented area far back on 

 the right side of Fig. 22 points to the beginning of restric- 

 tion between side patch and rump patch. The separation 

 of these patches by a transverse mid-dorsal break is 

 shown in Fig. 21, and their complete separation on the 

 left side appears in Fig. 23 (the transverse white mark), 

 while in Fig. 24, owing to the failure of the left-hand 

 pleural patch to develop, the two rump patches, both par- 

 tially separate from each other, are wholly disconnected 



