190 The Nature of Biological Diversity 



cal kinety fields. The kineties anterior to the cleavage line begin to 

 elongate before those posterior to the line; in this region, those on 

 the left of the oral meridian elongate more than those on the right. 

 Figure 12^4 shows an early stage of this process and Fig. 12B a late 

 stage. The result is to establish the typical posterior right and left 

 kinety fields between the old oral apparatus and the cleavage line. 

 The juncture of the two new kinety fields is the new postoral suture. 

 There the new cytopyge is destined to be formed, but it is not yet 

 present. 



As Fig. 12^4 shows, considerable kinety elongation occurs anterior 

 to the cleavage line before any can be detected posterior to the line. 

 Later, the kineties posterior to the cleavage line begin to elongate 

 (Fig. 12B). This takes place differentially, but in reversed relations; 

 i.e., the kineties on the animal's right of the new oral apparatus 



FIG. 12. Reconstruction of new kinety fields in the fission of singlets, doublets, 

 and one type of incomplete doublet. 



A. Singlet. New oral apparatus still at cleavage line. Between the level of the old 

 oral apparatus and the cleavage line, the kineties have elongated, more on the left 

 than on the right, thus starting the formation of new posterior right and left kinety 

 fields. 



B. Singlet. Anterior to the cleavage line, the two new posterior kinety fields 

 have almost been completed by further elongation of kineties, more on the left 

 than on the right. Cytopyge is not yet developed at the juncture of the right and 

 left fields (postoral suture line). At this stage, the kineties posterior to the cleavage 

 line have also begun to elongate, much more on the right than on the left, begin- 

 ning the formation of the new anterior right and left kinety fields. The right 

 kineties, by elongating up above that part of the cleavage line which is connected 

 with the anterior part of the new oral apparatus, are converting this part of the 

 cleavage line into the new preoral suture. 



C and D. Two focal levels of doublet in a stage of fission between that of the 

 singlets in A and B. C, surface at upper focal level; D, surface at lower focal level. 

 The areas on the sides of both oral meridians show the same developments as for 

 the areas adjacent to the single oral meridian in singlets. C shows the earliest 

 beginnings of the new preoral suture; i.e., the kineties to its right are just beginning 

 to elongate. 



E. Upper surface of an incomplete doublet showing the oral meridian that lacks 

 a vestibule, mouth, and gullet. Same stage of fission as the singlet in B; formation 

 of the preoral suture from part of the cleavage line by elongation of right kineties 

 up above part of it. 



Symbols for Figs. 10, 11 and 12: al, anterior left kinety field; ar, anterior right 

 kinety field; cl, cleavage line; cvp, contractile vacuole pore; fe, area of increase in 

 number of kinetosomes; fee, area of elongating kineties; m, mouth; mn, mouth of 

 new oral apparatus; mo, mouth of old oral apparatus; pi, posterior left kinety 

 field; pr, posterior right kinety field; py, cytopyge; pyo, cytopyge open (to dis- 

 charge undigested food) ; roa, rudiment of oral apparatus; s, preoral suture; scl, 

 preoral suture forming from part of cleavage line. 



