PHYSIOLOGY OF RECONSTITUTION OF PLANARIA LATA. 117 



dividuals averaging 12-14 mm., occasionally 16 mm. Body rela- 

 tively broad, in full-grown animals width from tip to tip of ce- 

 phalic lobes and from margin to margin at mouth about one sixth of 

 length. Pigment pattern coarse, irregular, often with obscure 

 dark median stripe, the general effect being mottled or dappled 

 light grayish brown. Cephalic lobes short. Animal sluggish and 

 markedly less sensitive to external factors than either P. doroto- 

 cephala or P. maculata. Structure of organ complex of genital 

 atrium as in Fig. 3. 



The present paper is primarily concerned with this species, but 

 attention is called to the physiological differences between this 

 species and P. dorotocepha which have been brought to light by 

 the experiments. 



The Occurrence of Fission and Sexual Maturity. 



Like P. dorotocephala, P. lata shows no visible morphological 

 indication of the presence of a posterior zooid, but, as will appear 

 below, the presence of such a zooid can be demonstrated by physi- 

 ological methods. Fission in P. maculata has been described by 

 Curtis ('02) and the act of fission has been observed in P. doroto- 

 cephala by Child ('10, '11c). In the latter species it consists in an 

 independent motor reaction of the posterior zooid while the animal 

 is moving forward. The posterior zooid attaches itself, the ante- 

 rior zooid continues to advance, and the body in front of the 

 attached portion is finally ruptured. Fission is much more likely 

 to occur after slight stimulation than after violent disturbance, for 

 in the latter case the posterior zooid is controlled by the anterior 

 and does not react independently. The act of fission has not been 

 observed in P. lata, but undoubtedly it occurs in the same manner. 

 In the laboratory fission is often induced by changing the water, 

 but does not occur at once, while the animals are very active, but 

 only later as their activity decreases. 



As regards the level of the body at which fission occurs, P. lata 

 differs markedly from P. dorotocephala. In the latter species fis- 

 sion normally occurs at a level 1-3 mm. posterior to the mouth 

 (Fig. 5), and in cases of delayed fission in the laboratory the 

 posterior fission piece may be longer than the anterior. In P. lata, 

 however, fission takes place much nearer the posterior end (Fig. 6) , 



