112 University of California Publications in Zoology [Vol.19 



posterior pole of the body of the parent, but no plasmotomy lias as yet 

 taken place. During this stage another set of intracytoplasmic 

 organelles, namely axostyle, anterior peristomal fibrils, and postero- 

 lateral flagella have been formed. The method of forming the axo- 

 style is one of splitting of the parent structure (pi. 1, fig. 3), but 

 there is no evidence at hand to determine the exact method of forma- 

 tion of the other organelles. 



In the colon and in the rectum occur the binary cysts (pi. 1, 

 figs. 7, 8) which represent the completion of binary fission. The 

 method of plasmotomy appears to be a longitudinal division of the 

 parent on a plane parallel to the major axis of the body and at first 

 horizontal, at least in the matter of nuclear separation, for even before 

 plasmotomy has begun (pi. 1, fig. 6) two of the nuclei and the axostyle 

 are seen to be upon a different optical plane from that of their sister 

 structures. The axostyle, however, appears to split in the sagittal 

 plane. When plasmotomy has been completed a side view of the two 

 flagellates within the cyst shows one individual above the other. Tin 

 line of separation of the two flagellates may have no reference to any 

 plane of plasmotomy. Kofoid and Swezy (1916) have shown that 

 movement in trichomonads is very active during plasmotomy in the 

 free state. It is quite possible that this also occurs within the cyst 

 in Giardia. An oblique view (pi. 1, fig. 8) shows the two flagellates 

 occupying such a position that their anterior ends are at opposite ends 

 of the cyst ; this is the position which they might assume if plasmotomy 

 occurred by a longitudinal cleavage of the body in a plane horizontal 

 to the major axis of the body. However, in typical binary cysts the 

 two zooids lie in an end to end, back to back position. 



It is noteworthy that in all the cj'sts of the binary type the indi- 

 viduals resulting from binary fission of the parent flagellate lack 

 their respective parabasal bodies. This absence has been shown by 

 the author (1917) to be correlated with the depletion of the reserve 

 material in the form of these bodies during encystment, and with 

 excessive mitotic and motor activity of the flagellate while in the free 

 state. These bodies at first hypertrophy during encystment, often 

 they are scattered like a cloud of chromotoidal substance in the cyto- 

 plasm (Kofoid and Christiansen, 1915), but are always absent in the 

 free somatella stage or when plasmotomy during binary fission within 

 the cyst has been completed. 



Infection is known to occur by the ingestion of the cysts ; no inter- 

 mediate host is necessary. The cyst wall in all probability is digested 



