A STUDY IN. POLARITY 167 
The nervous tissue, therefore, has the form of-a circular or oval 
ring. Miss Keiller (10) found a somewhat similar condition in 
the nervous system of heteromorphic heads of Planaria simpli- 
cissima, the old nervous tissue in this case being the brain. She 
Fig. 11 Photomicrograph of a frontal section of a triangular piece of Group D, 
showing nervous system in form of a ring surrounding tissue from the digestive 
organ (ef. fig. 10a and b). 
states, “The new brain is almost an exact counterpart of the 
old; and the two together form an almost circular structure.” 
Lang (712) and others (Flexner, ’98; Stevens, ’01; Shultz, ’02), 
as opposed to Lehnert (91) and Bardeen (’01), claim that in the 
regeneration of planarians the new nerve cord arises de novo 
in the new tissue at the ends of the old nerve cords, and never 
