186 The Nature of Biological Diversity 



vacuole pores and meridians, and various hereditary disturbances in 

 the development, growth, and relative movement of cortical parts 

 during fission. We shall not go into these in this chapter, although 

 each contributes further evidence of the genetic autonomy of cortical 

 parts and further insight into how cortical structures are reproduced. 



The features of cortical reproduction which hear importantly on 

 the present problems will become evident from a comparative exam- 

 ination of how the main components of the highly structured oral 

 segment are reproduced in singlets, doublets, and incomplete doublets. 

 Each of the structures of this segment — the vestibule, mouth, and 

 gullet of the oral apparatus and the sutures, the kinety patterns, and 

 the cytopyge — follows the same rules of reproduction in all three 

 kinds of cells. The differences among the cell types reveal what is 

 essential and what is nonessential for the reproduction of various 

 parts. 



We can undertake to give here only some of the salient features of 

 the reproduction of the parts of the oral segment. Actually the story 

 is still not fully known in spite of careful study by many investigators. 

 Among the early signs of reproductive activity, three are outstanding 

 (Fig. 10). (1) A field of new kinetosomes appears near the posterior 

 and right margin of the mouth (Fig. 10A ) . This is the rudiment of 

 the new oral apparatus, destined to pass to the opisthe, the posterior 

 product of fission. (2) Many new kinetosomes appear in the kineties 

 of the left wall of the vestibule and in a zone extending to the left 

 from the vestibule (Fig. 10B ) . (3) Within this area the kineties break 

 to form the cleavage line (Fig. 10B ) , which begins between the left 

 posterior edge of the old mouth and the left end of the rudiment of 

 the new oral apparatus. The cleavage line continues around the body 



FIG. 10. Formation of the oral rudiment in the fission of singlets, doublets, and 

 one type of incomplete doublet. Silver preparations. 



A. Singlet. Rudiment of new oral apparatus forming along the right side and 

 posterior end of the mouth and vestibule of the old oral apparatus. 



B. Singlet. Two focal levels showing the new oral rudiment which is beginning 

 to be shifted posteriorly by elongation of vestibular kineties. 



C and D. Two focal levels of doublet. C, surface at upper focal level; D, surface 

 at lower focal level. Both oral meridians and adjacent areas are undergoing the 

 same developments as the one oral meridian and its adjacent areas in the singlet 

 in B. 



E and F. Two focal levels of incomplete doublet lacking one vestibule, mouth, 

 and gullet. E, surface at upper focal level showing the oral meridian that lacks 

 vestibule, mouth, and gullet. A cleavage line extends from this meridian almost all 

 the way around the body. F, surface at lower focal level with complete oral merid- 

 ian; new oral rudiment extends slightly posterior to fission line. 



Symbols for Fig. 10 are given in the legend for Fig. 12. 



