136 University of California PubUcations in Zoology [Vol. 20 



When binary fission occurs in the free and encysted stages the 

 newly formed neuromotor apparatus, which is attached to the eentro- 

 some at one of the poles of the dividing nucleus and goes into one 

 of the daughter individuals, has the same asjonmetry as that of the 

 parent individual which goes into the other daughter. The structures 

 at tlie two poles are not mirror images each of the other. Bilateral 

 sjnnmetrj^ of the two as yet unseparated daughter cells is not estab- 

 lished by mitosis. 



The structures which we have discovered in the neuromotor organ- 

 ization of Chilomastix are, in detail, similar to and homologous with 

 those of the right cell in the binucleate Hexamitidae and most closely 

 with those of the cell on the right side of diardia. The centrosome, 

 nuclear rhizoplast, blepharoplast complex, peristomal fiber, parabasal, 

 and parastyle are homologous with the like organs in Giardia, save 

 that the parastyle is represented by the axostyle with its free terminal 

 flagellum in Giardia-, but has no fl2igellum in Chilmnastix. The three 

 anterior flagella of the former are represented by the lateral, postero- 

 lateral and ventral flagella of the latter, while the undulating flagellum 

 of the eytostome seems to have disappeared in Giardia. The asym- 

 metry of the cytostomal region of the two genera is of like pattern 

 and the spatial relations of this region to nucleus, centrosome and 

 blepharoplast are strikingly similar in the two organisms. 



It is obvious from the comparative morphology of these two genera 

 that they are rather closely related and that the Hexamitidae, includ- 

 ing Giardia, must be regarded as an offshoot from Chilomastix or its 

 near relations. Giardia differs from Chilomastix in two very impor- 

 tant particulars: (1) it is binucleate and (2) it is bilaterally sym- 

 metrical with the left cell the mirror image of the right, whicli is the 

 morphological equivalent of Chilom-astix. 



This binucleate bilateral organism could be derived from sinistral 

 Chilomastix only by a complete morphological reversal of the funda- 

 mental asymmetry of the left cell, with its nucleus and neuromotor 

 apparatus, in the binucleate body and the coincident suspension of 

 plasmotomy. Such a process, which is not unlike molecular reversal 

 in principle, might establish the bilateral binucleate Hexamitidae. 



The morphological relations of Chilomastix and Giardia are pro- 

 foundly significant and instructive as to the method by which bilater- 

 ality has appeared among the highly specialized, asymmetrical, uni- 

 cellular organisms. 



