CENTRAL NERVOUS SYSTEM IN PLANARIA. 55 



mon- -upport to this conclusion than to that view which maintain-' 

 i hat the regenerating nervous system grow- out anteriorly from 

 the cut ends of the old nerve cords. Apparently in tln-r cases 

 ,i neu central nervous system develops and is abnormal from 

 the lie-inning, but as its differentiation extend- posteriorly it 

 meet- the old nerve cords and unites with them. In this \\ ax- 

 il i- easy to account for the relatively sudden change in the 

 -iructure of the nervous system as we follow it posteriorly from 

 the ne\\ into the old tissue in some of the teratomorphic forms 

 Fij i } and Is, 26 and 27, 30 and 31). But ex'en the anterior 



ions oi the old nerve cords undergo reorganization to a greater 



or le-- extent under the influence of the nexv region anterior to 

 them. In bigs. 15, 27 and 31 they are nearer together and con- 

 m i ted 1.x larger commissures than they were ori-inallx- when 

 thex ii.nned a part of the posterior region of the In-t /ooid. 

 The structure of the nervous system in the posterior part of the 



ni-rate. 1 tejon in such cases as Fig. 14 and Fig. $<> suggests 

 a breaking up into separate IUTVL-S , but posterior to the lex els of 

 iln se -' tions where the developing jjortion meets the o'd cords 

 there i- a return to something approaching normal structure. It 

 is possible that if the old cords were not present in such cases 

 the m u IHTXOUS system would extend posteriorly as a c,,n-i( Id- 

 able number of separated cords or nerves instead of in the lorm 

 eharai leii-tic of normal animals. 



It \\as pointed out in Section I. that the teratophthalmic and 

 teratomorphic forms can be produced experinientallx bx de- 



!-ing the rate of the dynamic processes in the piece b. l..\\ ,i 

 n Main rale necessary for the production of normal animals 

 \\hich i> it-elf not constant but dependent upon various condi- 

 tions. |'he-e abnormal forms then represent planarian morpho- 



esis corresponding to certain rates ,.i reaction belo\\ the 

 " normal " rate for the existing conditions. The tact that change- 

 \\hich are primarily quantitative gixe rise to -uch dilleivnces in 

 structure a- those recorded is important. A- the rate <.t reaction 

 decreases we see certain parts, c. "... the preocnlar re-ion of tin- 

 head, decreasing in relative size and finally disappearing and 

 in the nervous system the bilateral structure of the ganglia 

 becomes lc-- and less distinct in consequence of the reduction and 



