188 The Nature of Biological Diversity 



and eventually ends in the right wall of the vestibule at the anterior 

 end of the rudiment of the new oral apparatus. 



Exactly the same processes occur in doublets, as appears in Figs. 

 IOC and 10D. What happens on the one oral segment of singlets hap- 

 pens on both oral segments of doublets. The same is true for the two 

 oral segments in incomplete doublets possessing two adjacent oral 

 segments but only one dorsal field. Of special significance is what 

 takes place in incomplete doublets that lack only the vestibule, mouth, 

 and gullet on one of their two oral meridians (Figs. 10E and 10F). 

 The complete oral segment behaves as already described for the oral 

 segments of singlets and doublets. The incomplete oral segment does 

 not form a rudiment of a new oral apparatus; but the other two 

 processes — increase in kinetosomes and formation of the cleavage line 

 — occur nevertheless. The cleavage line starts on the left of the de- 

 fective oral meridian at the place where the missing vestibule, mouth, 

 and gullet should be and passes to the left until it reaches the right 

 edge of the mouth on the complete oral meridian. And the cleavage 

 line that began on the left of that oral meridian passes to the left 

 until it meets the incomplete oral meridian on its right side. Hence 

 the increase of kinetosomes and formation of the cleavage line are 

 independent of the presence of vestibule, mouth, and gullet. And a 

 rudiment of a new oral apparatus appears where and only where an 

 oral apparatus already exists. Thus only one arises in singlets, two in 

 complete doublets, two in incomplete doublets possessing two com- 

 plete oral meridians, but only one in incomplete doublets possessing 

 an oral apparatus on only one of their two oral meridians. 



The main feature of reproduction of the oral segments in the next 

 stages in fission is elongation of the kineties in which new kinetosomes 

 were earlier packed close together. When the cleavage line reaches 

 the right wall of the vestibule, many new kinetosomes appear there. 

 The elongation of the kineties anterior to the cleavage line on both 

 sides of the old oral apparatus has the effect of pushing posteriorly 

 both ends of the rudiment of the new oral apparatus. This places the 

 new rudiment, which is organizing into an oral apparatus, entirely 

 posterior to the old oral apparatus (Fig. 11,4). Unequal growth places 

 the new rudiment to the right (Fig. 11B). The same elongation of 

 kineties occurs on both oral meridians of doublets; and wherever 

 there is a rudiment of a new oral apparatus, it is thereby carried 

 posteriorly and to the right. Figures 11C and 11D show this stage on 

 both oral segments of a complete doublet. 



The further elongation of kineties occurs differentially in groups, 

 according to a definite pattern which results in the formation of typi- 



