316 C. M. CHILD. 



largely at the expense of other parts, to use their substance in 

 maintaining its own structure. 



Only a few few brief suggestions as to the occurrence and general 

 significance of dominant and subordinate regions in organisms 

 are possible here. The dominance of the growing tip over other 

 regions in the plant is a well established fact: in my work on 

 Tubularia (Child, 'oja, b, c, d) I called attention to various facts 

 which indicate that the hydranth region is physiologically domin- 

 ant over other parts and the same is true for Corymorpha and many 

 other, perhaps all hydroids and for all actinians with which I 

 have worked. Moreover, when we recall that in most if not all 

 animals development of the egg begins at the animal pole 

 and that the distal or anterior region arises at or near this pole, 

 the possibility that the "animal" distal or anterior region is very 

 generally dominant becomes at once apparent. I believe that 

 we have here a very general law of development which has not 

 been clearly recognized. It is probable, however, that in many 

 forms the specification of different regions of the egg becomes 

 fixed to such an extent that their further differentiation is to a 

 greater or less extent constitutional rather than correlative, but 

 there are various facts which indicate that within each such 

 "self-differentiating" part we shall find a condition of dominance 

 and subordination of parts more or less similar to that which 

 exists in the whole egg and often even in the adult of some other 

 less highly differentiated forms. 



II. THE REGULATORY DEVELOPMENT OF THE ANTERIOR END. 



The "normal" head of P. dorotocephala possesses the form shown 

 in Fig. i . As in various other species of Planaria the eyes consist 

 of black pigment spots and unpigmented sensory areas. The 

 sensory cephalic lobes or auricles are also only slightly pigmented 

 and are marked off as well-defined light areas from the rest of 

 the dorsal surface of the head, which is dark brown. The light 

 areas of the eyes and the auricles are indicated in Fig. I by dotted 

 lines. 



The result of regulation in the larger isolated pieces is usually 

 a "normal whole" (Fig. 4), i. e., and individual with head essen- 

 tially like Fig. i, though varying in size, and the other organs 



