1915] Kofoidand Christiansen: On Giardia muris (Grassi) 43 



is perhaps to be seen in plate 7, figure 31. A completed 8-niieleate stage 

 is shown in figure 48, and the nuclei of this are in the prophase for 

 the next division. A group of four small individuals probablj' in a 

 late stage of disintegration (pi. 7, fig. 49) of the plasmodium were 

 found which have about the size that two have in the preceding stage 

 (fig. 48). They suggest the completion of the 8-zooid. 16-nucleate 

 Plasmodium or somatella, and its subsequent disintegration into single 

 binucleate individuals of small size. 



The small free zooids resulting from multiple fission are not to be 

 confused with the small Hexaniitus miiris Grassi (see Wenyon, 3907) 

 which occurs in the same hosts with Giardia muris. It is, as Wenyon 

 (1907) has stated, a distinct species. Hartmann's (1910) .suggestion 

 that it is a phase of the life history of Giardia muris is entirely un- 

 supported by our observations on the life-history of Giardia and the 

 structure of Hexaniitus muris. 



This method in which the form of the original parent individual is 

 lost after the first mitosis is not, however, the only method of formation 

 of a multinucleate somatella. Another type (pi. 7, figs. 45-47) occurs 

 among non-encysted individuals, in which the parental form is pre- 

 served during the multiplication of nuclei, and the extranuclear 

 neuromotor apparatus does not keep pace in its divisions with nuclear 

 duplications, there being for example eight nuclei and but two axo- 

 styles, and not as yet two full sets of fiagella. 



An individual which has reached the 8-nucleate stage (4-zooid) 

 is shown in plate 7. figure 45, and in the prophase of the next division 

 leading to the 16-nucleate (8-zooid) in plate 7, figure 46. The nuclei 

 are symmetrically arranged in the second, but are scattered without 

 reference to bilateral symmetry in the first. In figure 47 we have 

 apparently a moribund individual approaching the 8-nucleate stage, 

 in which the cytoplasm is full of deeply staining chromidial masses 

 and the nuclei are passing through the second division leading to the 

 8-nucleate plasmodium. It shows two nuclei in recent constriction 

 and two not yet constricted. Evidences of four chromosomes are 

 present in figures 45 and 46. The main distinction between this method 

 of multiple fission and the one first de,seribed lies in the delay in the 

 division of extranuclear organelles in the latter. The individuation 

 of the potential organisms is less advanced in the cytoplasmic region 

 than it is in the nuclear, pos.sibly as the result of external conditions 

 acting on one or both these regions. 



