OLIVE: MONOGRAPH OF THE ACRASIEAE. 
459 
condition, they become heaped together into colonies, thus forming 
characteristic aggregations to which the name “ pseudoplasmodium’’ 
has been applied. 
The period of vegetation includes that stage of the developmental 
history during which the myxamoebae absorb nutriment, increase in 
size, and become greatly multiplied by successive division. The 
morphology and the development of the individuals during this con¬ 
dition have been studied in special detail in Dictyostelium and in 
Polysphondylium, which possess features in their vegetative condi¬ 
tions that appear to be identical. To Brefeld in particular, is due 
the credit for our present knowledge of the details of the structure 
and development of these two members of the group, as van Tieghem 
and others confined their studies rather to superficial characters. 
I have supplemented Brefeld’s observations with others dealing 
principally with the minute structure of the myxamoebae and have 
been able also to correct certain conclusions held by several investi¬ 
gators who have studied the group. 
I have given special attention also to the vegetative individuals of 
Dictyostelium and Polysphondylium, and have tried to follow out 
and explain the nuclear changes which have been observed through¬ 
out the life history of the organisms; the whole matter, however, 
remains as yet in doubt. The living organisms have been studied 
throughout the entire cycle of development in hanging drop cultures, 
and these observations have been supplemented by studies of individ¬ 
uals killed and stained at every stage of development. 
Nutrient media of various kinds, among which a weak horse dung 
decoction and a peptone culture medium w^ere found to be the most 
satisfactory, were added to 1.5% to 2.5% of agar, then filtered and 
sterilized. Of the various cells employed for drop cultures, one 
which furnished perhaps the most favorable conditions for prolonged 
study consisted of a small, shallow stender dish of one inch or more 
in diameter, containing a small amount of water and covered with 
a thin cover glass, cemented to the rim. Sufficient light for the 
examination of the myxamoebae with high powers was obtained by 
concentrating, by means of a bull’s-eye lens, the rays from a Wels- 
bach gas lamp upon the mirror of the microscope. In this way only 
could enough light be obtained during the employment of immer¬ 
sion lenses and oculars giving magnifications of 2,000 and more 
diameters, which was necessary to secure any appreciable differenti¬ 
ation of the cytoplasmic contents. 
