284 C. M. CHILD. 



In another series pieces comprising the whole post-pharyngeal 

 region were used (posterior to d, Fig. i). These pieces, after 

 eighteen days in 0.5 per cent, ether, had attained the condition 

 shown in Fig. 14. A small head is forming, almost entirely by 

 redifferentiation, at the anterior end, with one median eye, and a 

 small pharynx is present. At this time half of the pieces were 

 returned to water and half remained in ether. After nine days 

 more the pieces in water had acquired the condition shown 

 in Fig. 15 while those in ether were like Fig. 16. In the pieces 

 returned to water the anterior half is greatly elongated but the 

 posterior half remains much as before. In these pieces, as in the 

 one described above, the posterior part was dragged about by the 

 more active anterior portion. Gradually complete coordination 

 returned and the posterior end began to attach itself to the sub- 

 stratum as the animal advanced and after this the shape gradually 

 approached that of the normal animal. 



Further data along this line could be presented but these cases 

 are sufficient to show that it is possible to delay or inhibit the 

 change in shape to any desired degree, and to induce its occur- 

 rence at any time. Moreover, the manner in which it occurs 

 can be controlled and altered indirectly by using pieces of dif- 

 ferent sizes and from different regions of the body. 



To my mind the evidence indicates very clearly that the change 

 in shape is primarily a mechanical deformation of the body in con- 

 sequence of the altered direction of the strains to which it is sub- 

 jected as the animal advances. To control the change experi- 

 mentally we have only to control the locomotor activity. In 

 several of my earlier papers (Child, '02, '03, '04^) I have de- 

 scribed the method of locomotion in certain species of turbel- 

 laria : in Planaria longitudinal strain arises in essentially the same 

 manner as in the other forms discussed, the use of the posterior 

 region as an organ of attachment being one of the chief factors. 

 Moreover, there is considerable direct evidence that the tissues of 

 a region undergoing change of shape are being stretched. In 

 regions where the decrease in width and the elongation of the old 

 parts begin, the chromatophores are always greatly elongated, 

 and their elongation is greatest where the change in shape is most 

 rapid. As the change goes on they become drawn out into long 



