214 



DYNAMICS OF LIVING MATTER 



H 



H 



the experimenter has it entirely in his power to determine whether the 

 Crustacean shall regenerate an eye in the place of the eye which has 

 been cut off, or an antenna. It depends upon the fact whether or not 

 in the operation the optic ganglion is removed with the eye. If the 

 optic ganglion is removed with the eye, an antenna is regenerated in 

 the place of the eye. If the optic ganglion is left intact, a new eye is 

 formed. These experiments were carried on successfully in Palaemon, 

 Palaemonetes, Sicyonia, Palinurus, and other Crustaceans. Herbst says 

 that the optic ganglion exercises a "formative stimulus" upon the 

 hypodermic cells of the wound. It is certain that an explanation of 

 the role of the ganglion can only be given in physical or chemical 

 terms; that as long as this is not possible we possess no explanation. 



Morgan has made a somewhat similar observation on earthworms, 

 in which he found that a new head is only possible at the anterior cut 



end of the nerve cord.* The following 

 case may be mentioned by way of illus- 

 tration. A few of the anterior segments of 

 an earthworm were cut off, H, Fig. 57, and 

 from the remaining body a piece ab was 

 cut from the anterior part of the nerve cord 

 (see Fig. 57), while all the other tissues 

 remained unaltered. The anterior cut end 

 a of the worm healed, and no new head 

 formed at this place. Instead, a new head 

 was formed in some such cases at b, at the 

 anterior cut end of the nervous system. If 

 the head alone is cut off in an earthworm 

 without the excision of the anterior piece 

 of the nerve cord, a new head is formed at 

 the anterior end of the body. In another 

 series of experiments Morgan cut off the 

 head H of an earthworm and in addition 

 (Fig. 58) excised a piece be from the nerve 

 cord, so that now two anterior cut ends, a, 

 and c, Fig. 58, of the nerve cord existed. 

 In a few of these cases two new heads were 



FIG. S7- FIG. 58. - , i r ,1 j r 



AFTER MORGAN. AFTER MORGAN, formed, one at each of the anterior ends 01 



the nerve cord, at a and at c. 



Another example of the dependence of the regeneration of one organ 

 upon the presence of another is found in the formation of the lens of 

 the eye. As is well known, the formation of the lens in the eye is pre- 



* Morgan, Regeneration, p. 52, New York, 1901. 



m 



