ZOOLOGY: KOFOID AND CHRISTIANSEN 
547 
ON THE LIFE-HISTORY OF GIARDIA 
By Charles Atwood Kofoid and Elizabeth B. Christiansen 
ZOOLOGICAL LABORATORY. UNIVERSITY OF CALIFORNIA 
Presented to the Academy, September 1 5, 191 5 
This paper is based upon the examination of 220 mammals, gray and 
white culture mice (98), culture rats (42), and wild Peromyscus manicu- 
latus gamheli (Baird) (59), Microtus calif ornicus calif ornicus (Peale) (6), 
and coyotes [Canis ochropus ochropus (Peale)] (2) trapped near Berkeley. 
About 13% of the total were infected, 22 of the mice, and 4 of the 
Peromyscus y with Giardia muris and three of the Microtus with G. microti 
sp. nov. Several hundred slides prepared by the Schaudinn-iron haema- 
toxylin wet method were examined and stages in binary and multiple 
fission and in encystment obtained in quantity. 
In the last edition of Kolle and Wassermann's Handbuch der pathogenen 
Mikroorganismen Jollos (1913) denies the existence of multiplicative 
phases in Lamhlia in the free stage in the intestine of the host. Roden- 
waldt (1911) had also held this view in Prowazek's Handbuch der patho- 
genen Protozoen and Alexeieff (1914) and Prowazek and Werner (1914) 
as the result of recent investigations have confirmed it. Castellani and 
Chalmers in the last edition (1913) of their Manual of Tropical Medicine 
accept Noc's (1908) conclusion, seemingly made without study of com- 
parative material, that there is only one species concerned in the intesti- 
nal infections alike of mice, rats, cats, and man, and make the infer- 
ence that such vermin become sources of infection to man by the con- 
tamination of his food and water with their cyst-bearing faeces. 
It comes thus to be of interest to human and comparative .medicine 
to learn whether or not these intestinal flagellates which produce a 
chronic enteritis in young mice and are under suspicion of similar ac- 
tivity in man, may multiply in the free state in the intestine, or whether 
infection is limited to the number of ingested spores, and especially to 
learn whether there are separate species restricted to given host . ecies 
or groups of species of rodents, and another species found in man. 
If the latter be the case, the probability of these mammals being sources 
of infection is reduced considerably, unless it be that the parasites 
transform from one ^species' to another with change of host. The bio- 
logical problems of host specificity, and transformation by environment 
are thus involved in this question. 
It is the purpose of this paper to demonstrate the occurrence of binary 
and multiple fission in Giardia { = Lamblia), in both the free (hitherto 
denied) and encysted stage, to describe mitosis in these minute organ- 
