PHYSIOLOGY OF RECONSTITUTION OF PLANARIA LATA. 119 



observed in P. lata, but the susceptibility (p. 32) and the head 

 frequency (p. 37) of the region just anterior to the level of fission 

 suggest that some slight degree of physiological isolation and the 

 earliest stages of another zooid exist there in large animals. If 

 such a zooid is present, it develops to a stage at which fission is 

 possible only after fission has occurred posterior to it and it has 

 become the posterior end of the body. 



The maximum length attained by P. lata under ordinary condi- 

 tions in nature and in the laboratory is twelve to fourteen milli- 

 meters, and at this size the animals become sexually mature, even 

 though fission continues to occur. The level of fission is so far 

 posterior to the genital pore that the development of the ducts and 

 pore is usually not affected to any appreciable degree by fission. 

 In this respect also this form differs from P. dorotocephala, in 

 which the level of fission is so near the level of the genital pore 

 that the occurrence of fission in an animal approaching sexual 

 maturity usually brings about disappearance of the pore and at 

 least the posterior portions of the ducts. In the localities about 

 Chicago P. lata becomes sexually mature and deposits eggs from 

 June to September. In the laboratory maturity and egg laying 

 may occur at any time of year. In P. dorotocephala sexual ma- 

 turity may occur in the laboratory when the animals are well fed 

 and fission is prevented by keeping them on slimy or vaselined 

 surfaces, but it has not been observed under natural conditions in 

 this region, being apparently prevented by fission and perhaps also 

 by periodic starvation (Child, '11c). Curtis also found that in 

 some localities P. maculata does not become sexually mature, but 

 did not discover the determining conditions. It may be suggested 

 that an environment which inhibits slightly the physiological ac- 

 tivity of the animals and so decreases the range of dominance may 

 result in the occurrence of fission at a more anterior level, and this 

 may interfere with the development of the genital ducts and pore. 

 Experiments to test this suggestion have not yet been performed. 



Methods. 



The animals are found both on stones at the bottom and on 

 Elodea and other water plants at various levels. On the stones all 

 sizes from very young to sexually mature animals and numerous 



