i 919 ] Boeck: Studies on Giardia Microti 111 



flagellate characteristic of its free state, the only exception being 

 the absence of the extracytoplasmic parts of the anterolateral, postero- 

 lateral, and caudal flagella. The intracytoplasmic portions of these 

 last named structures are still present within the cyst. 



In the metamorphosis of a single individual cyst into a binary 

 cyst the nuclei undergo division. Distinct anaphase spindles were 

 found in many cases (pi. 1, fig. 3), but no late prophase, metaphase, or 

 telophase stage was found. The mitotic activity during these stages 

 is probably very rapid while the anaphase is of longer duration. 

 The telophase was detected in cysts of Giardia found in the rats 

 (pi. 1, fig. 12). In no instance was the chromatin of either nucleus 

 seen to have resolved itself into chromosomes. Centrosomes were dis- 

 tinguishable in some of the cysts (pi. 1, figs. 2, 4), but in no case were 

 they found to be divided with one daughter centrosome located at 

 each pole of each nucleus. 



In the cyst shown in plate 1, figure 1, the nuclei of the organism 

 appear to be in the resting stage, for the karyosome is a single ovoid 

 mass of chromatin. This stage in the development within the cyst 

 is very typical of the nuclear constitution of all the single-individual 

 cysts found in the ileum and the caecum. 



In the caecum single-individual cysts were also found, which, 

 however, present the first stage of definite progressive development 

 beyond that described in the preceding paragraph. These cysts 

 (pi. 1, figs. 2^) show the chromatin of the nuclei dividing or already 

 divided into two separate masses upon a spindle, with evidence of their 

 migration to the poles of each nucleus. This stage corresponds to the 

 anaphase in mitosis. Passing along the digestive tract from the 

 caecum to the colon we find a few of the single-individual cysts are 

 still to be found, even as far down as the rectum, but their number 

 is very small. 



In the colon the next stages of development within the cysts were 

 found. The nuclei have divided to form four daughter nuclei (pi. 1, 

 fig. 5), which are very close to each other and in the resting stage. 

 The cytoplasmic body within the cyst is still in the condition of a 

 single encysted individual, no plasmotomy having as yet taken place. 

 The axostyle has partially divided, but only two distinct parabasal 

 bodies are to be seen. 



The next stage in the progress of binary fission within the cyst 

 is also to be found in the cysts in the colon. In this stage (pi. 1, fig. 6) 

 two of the nuclei have migrated to a position which was earlier the 



