GENUS: GIARDIA 



697 



division forms of the unencysted flagellates, while Kofoid and Chris- 

 tiansen (1915, 1915a) described a similar process of binary fission in 

 G. microti and G. muris. It seems evident that binary fission may occur 

 in the free condition, though most usually it takes place within the cysts, 

 which are passed in large numbers in the fseces of infected individuals. 

 The flagellates undoubtedly multiply in the intestine, and unless binary 

 fission in the unencysted stage takes place more frequently than has beeu 

 observed, it has to be assumed that the two flagellates which have resulted 

 from division within the cyst are able to escape from the cyst without it 

 leaving the host, a condition of affairs which is quite exceptional for 

 intestinal Protozoa. In the case 



of most Protozoa, the encysted 

 forms are destined to escape from 

 the host in order to ensure infec- 

 tion of others. 



The stages of division of G. in- 

 testinalis are shown at Fig. 292. 

 It will be seen that the nuclei have 

 divided, and that there has been 

 duplication of the sucking disc 

 and various blepharoplasts, axo- 

 nemes, and flagella. The body 

 finally divides from before back- 

 wards. The details of the process 

 have not been worked out. It is 

 evident that division within the 

 cyst takes place in a similar 

 manner, but here the various du- 

 plicated structures are so crowded 

 together that it is impossible to 

 follow the details with any degree 

 of accuracy (Fig. 293). 



The flagellate encysts in an ovoid cyst which forms first around the 

 anterior end of the body. It extends backwards and gradually encloses 

 the tail, which is finally retracted within the cyst. In recently encysted 

 individuals the flagella and tail may be seen to be moving within the cyst. 

 In stained specimens it will be noted that the two nuclei move to the 

 anterior end of the body, where they divide to form four spherical nuclei. 

 The fibre which forms the margin of the sucking disc becomes duplicated, 

 and the two are often coiled in various ways. The blepharoplasts have 

 each divided, and new axonemes and flagella have been developed, so that 

 the cyst encloses an ovoid mass of cytoplasm containing four nuclei and 



Encysted 

 Intestine 



Fig. 293. — Giardia intestinalis : 

 Forms from the Human 

 (x 3,000). (Original.) 



1. Form with two nuclei. 

 2-.5. Forms with four terminal nuclei. 



6. Form in which two of the nuclei have 

 migrated to the opposite pole and the 

 flagellate is dividing within the cyst. 



