212 University of California PuMications in Zoology [Vol. 20 



The peristomal fiber is continuous across the median line in the 

 anterior arc, though at that point in some individuals it maj' be reduced 

 somewhat in size and degree of stainability. 



The peristomal fiber of G-iardia is the homologue of two peristomal 

 fibers of the trichomonads. Its dual origin is not sho^^•n on the anterior 

 margin but is preserved in the posterior one. The right half is an 

 identical homologue of the well-developed, sucker-like peristome of 

 ChiloniMstix and Waskia and of the feebly developed one of Tricho- 

 monas, Tritrichomonas, and Pentatrichomonas. The left half is a 

 reversed homologue, being a mirror image of the right. As in Chilo- 

 niastix the right half lies at the right of the axostyle and near a curved 

 parabasal, and a blepharoplast lies to its left. 



The function of the peristomal fiber is iinknown. It lies in the 

 margin of a mobile organ of adhesion in the part in closest contact 

 with the cells of the host, and in an area of maximum mobility in 

 contraction in the adliesive functioning of the cytostome. In so far as 

 its morphological relations indicate its function, it may be either 

 contractile or eonductile (sensory?) or possibly both in a primitive 

 undifferentiated state. 



We have thus seen tliat the fibrillar system joining karysome, 

 eentrosome, blepharoplast, cytostome, parabasal, flagella, and axostyle, 

 and connected across the median plane by the anterior node, trans- 

 verse commissure, and anterior peristomal fiber, forms a structurally 

 integrated whole of the organs concerned in locomotion and adhesion. 

 "We will now consider their relations in mitosis. 



iliTOSis IN THE Active Phase 



Owing to the fact that active flagellates are discharged in the faeces 

 under unusual conditions in the bowel from the site of the infection 

 at some distance up the intestine where multiplication by binary 

 fission is presumed to take place normally, the normal condition of 

 the chromatin is subject to modifications due to the moribund state of 

 the flagellates. The central karyosome is usually rounded up and 

 mitotic figures are generally rare in the free stages obtained from 

 stools. 



The following facts ai"e established by available data. Fir.st. mitosis, 

 and presumably its culmination in binary fission, takes place in the 

 free stage in the bowel (pi. 23, figs. 4, 5). Secondly, there are four 

 chromosomes, appearing as rounded-up spheroidal masses (pis. 23 and 



