Ellis. — The Life-history of Bacillus hirtus. 
247 
In the case of individuals in which young spores are present, the 
cytoplasm still stains deeply, but in most cases when the spores are fully 
ripe, as explained later, the cytoplasm stains only very lightly. In normal 
development, the number of spores entirely free from the vegetative cells 
which formed them is very small, though if the culture be well advanced the 
number may be considerable. 
In a four days old culture there will usually be seen free swimming 
vegetative cells without spores, spore-containing individuals which may or 
may not be at rest, and free spores. If the culture be well advanced 
the number of free spores will greatly preponderate, and in many 
cultures of this age scarcely anything except free spores is to be seen. 
In some cultures, however, there may be very few spores formed. It is 
difficult to account for this, for there is no apparent difference between 
these cultures and those in which spore-formation has been very active 
either in appearance or in the conditions of growth, but probably an 
important factor is that the amount of excreted matter is less in the latter 
case, thus not preventing the normal development of spores. In non- 
sporing cultures it is noticeable that the vegetative growth is much greater, 
which naturally infers a greater quantity of excreted matter. On the other 
hand it is remarkable that by withdrawing a culture from its own excretion 
products by reinoculating into another tube it is possible to postpone the 
formation of spores, so that it seems as if both the absence of excretion 
matter and the excess of it are detrimental to spore-formation. It seems 
as if unfavourable conditions, e. g. the presence of much excreted matter, 
induced a tendency to form spores in normal cultures, but if the unfavour- 
able conditions had gone too far, that the organism was not able to do this. 
There are doubtless other factors which influence the course of the life- 
history, but at present they are very insufficiently worked out. The same 
remarks apply to the motility of four-day-old, as to three-day-old cultures, i.e. 
the motility is dependent on the spore-formation. In Fig. 20, which was taken 
from such a culture, it will be seen that some individuals are still in the 
same condition as those in one-day-old cultures. 
From now on, no change takes place in the appearance of the growth 
on the surface of the Agar, except that due to the gradual evaporation of 
the water and the consequent drying up of the nutrient medium 
which slightly alters the appearance of the surface growth. The course is 
now complete and no further change takes place till the spores, finding the 
necessary conditions of temperature and moisture, begin the cycle once 
more by germinating. 
s 
