Role of Preformed Structure in Cell Heredity 193 



the preexisting vestibule, mouth, and gullet. We have not previously 

 mentioned this invagination process, which has been noted by Ehret 

 and Powers (1959), and the other details of the transformation of 

 the rudiment into a functional normal oral apparatus because much 

 is still obscure. But invagination of the gullet, the rudiments of which 

 develop in the cortex near the old mouth-vestibule juncture, is part 

 of the story. This invagination may also be correlated with elongation 

 of kinetics, possibly, at least in part, those of the rudiment itself. 



The other main kind of failure is the failure of the rudiment of 

 vestibule, mouth, and gullet to form and develop even when a pre- 

 existing oral apparatus is present. Two examples can be cited. First, 

 it can happen in complete doublets and it thus results in the origin of 

 incomplete doublets. Significantly, this failure is preceded and accom- 

 panied by certain other aberrations. The number cf kinetics between 

 the oral segment and the contractile vacuole pores to its right is much 

 reduced, from about 25 to about 8 to 15. The preoral suture is much 

 shorter and correspondingly the vestibule, mouth, and gullet are 

 much more anterior; i.e., they are located at its base. In correlation, 

 the postoral suture is much longer than normal. The cytopyge is more 

 variable in size, often being short and much further than normal 

 from the posterior end of the preoral suture. When these relations 

 exist, the vestibule, mouth, and gullet on such an oral meridian are 

 sooner or later destined to be irreversibly lost in the progeny. The 

 dislocation of parts that precedes and accompanies this loss suggests 

 an essential determinative role of localized regional interactions in 

 the formation of the oral apparatus. To this we return later. 



The second example is failure of incomplete oral meridians, and of 

 one of two closely placed oral meridians, to persist and be reproduced 

 at autogamy and conjugation. At present this is just a brute fact 

 which has not been studied as to details, even to the extent of knowing 

 whether it invariably happens. No such failure occurs when two com- 

 plete oral meridians are far apart. 



A different type of failure appears in some cells of doublets that 

 have two oral meridians less than 90° apart. At fission, the cleavage 

 line may be inhibited or appear late in the region between the two 

 mouths (Fig. 13.4). Correlated with this, the kineties in that region 

 often fail to elongate to the normal extent. Consequently, in the course 

 of successive fissions, the number of kineties between the two oral 

 meridians is progressively reduced, sooner near the poles than near 

 the equator, bringing the polar ends of the anterior and posterior 

 sutures together further and further toward the equator (Fig. 13,4 ) . 

 Eventually the two preoral sutures are reduced to one in this way 



