26 PATTERNS AND PROBLEMS OF DEVELOPMENT 



higher mean fission rate; in one rates were equal, and in another the rate 

 was higher in posterior selections. These data, particularly those of Pe- 

 tersen, suggest an anteroposterior differential in physiological condition 

 but indicate that, although the differences in the products of division may 

 be inherited, they do not necessarily persist indefinitely. 



A more or less complete replacement of peristome and its membranelles 

 and of the cirri occurs in both members in fission of hypotrichous ciliates. 

 The new cirri appear in certain areas or fields before the old cirri disap- 

 pear. In Euplotes the primordium of the peristome is the first indication 

 of reorganization of the posterior product of fission, the primordia of the 

 cirri appearing somewhat later (Wallengren, 1901). This suggests a gradi- 

 ent in the developmental field of the posterior member. From certain of 

 his figures {H and J) of Stylonychia it appears that the new cirri of the 

 anterior member are slightly more advanced in development than those 

 of the posterior member. If this is true, it suggests an anteroposterior 

 gradient in the parent individual. 



The gradient of the parent body is clearly evident in the successive 

 fissions constituting strobilation of the scyphistoma of Amelia and other 

 discomedusae. The longitudinal fissions which occur occasionally in hydra 

 and actinians, either as the result of injury (Roudabush, 1934) or other 

 conditions, begin apically and progress basipetally. Fission also occurs in 

 many flatworms, nemerteans, and annelids. In some cases it is essentially 

 fragmentation without definite fission zones, as in some nemerteans and 

 annelids. In others fission occurs at definite body-levels, and visible re- 

 constitution may precede or follow separation. Reconstitution of a whole 

 from the part in connection with fission in these forms is essentially simi- 

 lar to reconstitution following experimental isolation of pieces by section, 

 which is discussed below. Usually a part of the parent pattern persists in 

 the fission piece and becomes the starting-point for orderly reconstitution. 

 Fragmentation of nemerteans, Lumbriculus, and other forms apparently 

 results from strong muscular contraction without predetermination of 

 body-level. Separation at a definite body-level precedes morphological 

 development in Dugesia ( = Euplanana) dorotocephala and some other pla- 

 narians; but the new developmental pattern is physiologically distinguish- 

 able (pp. 108-17), and separation occurs when its physiological isolation 

 becomes sufficient to permit an independent motor reaction. In many 

 forms development precedes separation so far that chains consisting of 

 several zooids may result, separation occurring when head and posterior 

 end of adjoining zooids attain a certain degree of development. Figure 10 



