192 The Nature of Biological Diversity 



elongate much more than those on its left. The result (Fig. 12B) is to 

 establish the typical anterior right and left kinety fields in an area 

 of new growth between the new oral apparatus and the main part of 

 the cleavage line. I had to say "main part" because, as appears in 

 Fig. 12J5, part of the cleavage line — the part that was at first immedi- 

 ately on the left of the oral meridian — becomes the new preoral 

 suture. This happens as a consequence of the growth of the right 

 kineties up and to the left, so that they come to lie anterior to the 

 part of the cleavage line which is still connected to the anterior edge 

 of the new oral apparatus. 



Exactly the same processes occur in doublets and incomplete doub- 

 lets wherever there is an oral meridian. Figures 12C and 12D show 

 them occurring on both oral meridians of a doublet in a stage of 

 fission intermediate between that of the singlets of Figs. 12,4 and 12B. 

 Figure 12C is a particularly good picture of the beginnings of the 

 elongation of the right kineties posterior to the cleavage line: they are 

 already penetrating into the cleavage line and marking off the earliest 

 trace of the prospective new preoral suture. Figure 12E shows that 

 these definite patterns of kinety elongation take place around the oral 

 meridian even when it lacks a vestibule, mouth, and gullet. The new 

 preoral suture line forms posterior to the cleavage plane as a result 

 of the elongation of the right kineties into the cleavage line and 

 around that part of it destined to become the preoral suture. 



Thus far only the normal or regular processes in singlets, doublets, 

 and incomplete doublets have been described. They reveal much about 

 how cortical cell heredity is accomplished. More is revealed by ob- 

 servations on exceptions to this course of events, by failures of certain 

 processes to occur, and by the origin of certain structures under con- 

 ditions not expected from the preceding account of normal events. 

 Only some of the more important exceptions will be mentioned in this 

 paper. 



Two principal kinds of failures in the occurrence of expected 

 processes have been noted. On the one hand, certain groups of kineties 

 that normally elongate have failed to do so or have done so to a less 

 than normal extent. In some cases, the vestibular and nearby kineties 

 have failed to elongate and, as a consequence, the rudiment of the 

 new oral apparatus fails to grow apart from the preexisting oral 

 apparatus. This may be dependent upon the prior failure of new 

 kinetosomes to appear in these kineties; but the cause of these failures 

 is still obscure. Often, but not always, correlated with but slight 

 elongation of these kineties is failure of the new gullet to invaginate: 

 its kineties remain up on the body surface more or less posterior to 



