CENTRAL NERVOUS SYSTEM IN PLANARIA. 43 



but the length of time necessary for regulation in such cases is a 

 disadvantage. Abnormal heads produced by the action of anes- 

 thetics and by various other conditions were not included within 

 i!i- scope of the present investigation. It is not improbable that 

 compari-on of the heads produced by different conditions will 

 -how more or less characteristic differences in the nervous system. 

 In all ca-es described the pieces were kept for at least two 

 ueek- after -ection. After this length of time the new head is 

 well de\ eloped and those cases in which the teratomorphic head 

 does ii"! UMII, iin teratomorphic but redifferentiates into a head 

 ot normal -hape have already undergone this further regulation 

 or -lio\\ unmistakable indications of it. The teratomorphic 

 head- \\liiili persist as such for two weeks at 20 ('. almost 

 -hou any further changes. 



II I in MEAD OF THE NORMAL ANIMAL. 

 The form and the chief external features of the normal head 

 ot J'laiHiriti dorotocephala are .,ho\vn in Fig. i. The unpigmented 

 areas ! the eyes and the very slightly pigmented sensorj regions 



ol iln- auricle- are indicated by dotted lines. Except t"i" these 

 the dor-al .-mtace of the head is usually rather deeply ami uni- 

 loimly pigmenied. I mm the ventral surface the outline- "t 

 the (ephalic ganglia and nerve cords are in li-iin. il\ \i-ible in 

 the li\ in:: animal. 



I j 5 shou transverse sections ol the ner\ou- system 



ai ilillerent levels of the head region. 1 i^. J i- I' mm a K-\el 

 al nit half uav between the eyes and the lip of the head and 

 four nerves extending to the anterior head region. Further 

 these ner\es break up and become less di-tin<t as 

 the\ are ili-nibuted. l ; ig. 3 shows the icanulia at about one 

 fourth ot the distance from the e\ e- to the tip. The\ consist 

 of hlier tracts including a few i ell- .md -urmnnded \>\ maii\ 

 oilier-. At thi- level the chief fiber tract shou- indication- of a 

 beginning -eparation into right and left hal\e-. < )n e.ich side 

 of the chief tract is a small tract separated from it by cells: 

 the-e tuo -mall tracts are cros- .-ections of ner\e- \\hich pass 

 io the anterior re-ion- ol the head. 



i i-me 4 shows th ^ Jia at the level of the eyes. The t\\o 



