2 5 I 
Ellis . — The Life-history of Bacillus hirtus . 
clear space a round spot was visible, from which in one or two cases slight 
radiations were observable (Fig. 36). This I take to be the nucleus, 
surrounded by cytoplasm, which thus makes it a fairly prominent object. 
I did not see a single case in which more than one of these round objects 
was visible, and the method of preparation of the slide precludes the idea that 
they are artificial products. They are not always found at the centre (Fig. 
37). In Fig. 38 we see two examples of this stage in the same individual. 
It seems, therefore, that the first stage of development in the formation 
of the spore consists in the withdrawal of the bulk of the cytoplasm from 
a clear space. In this space is found a nucleus, which is surrounded by 
a thin film of cytoplasm, and connected to the cytoplasm surrounding the 
clear space by thin strands of cytoplasm. Fig. 39 represents the probable 
construction of the cell at this stage. We have therefore before us an 
instance of free cell-formation. The next stage is somewhat difficult to 
obtain. A change has taken place in the staining capacity of the young 
spore. As is seen in Fig. 40, the young spore stains more deeply than the 
surrounding cytoplasm, having evidently absorbed unto itself those materials, 
cytoplasmic and reserve, which are necessary for its construction. It is not 
possible now to see the nucleus. In a still later stage the staining capacity 
of the young spore is still greater, as evidenced by its deeper colour, and by 
careful observation and the proper amount of staining, the coats of the 
spore become marked off at the periphery by their denser colour (Fig. 41). 
In the fourth stage we see a change in the staining capacity of the spore, 
for the spore-coats being now dense, they prevent the access of the stain. 
This results, therefore, in the spore again becoming less deeply stained than 
the surrounding cytoplasm, and it gradually assumes the strongly refractive 
appearance characteristic of mature spores, giving each spore the appearance 
of a bright oval speck of light. To examine this stage a more concentrated 
solution of the stain must be used than the one given above, in order that 
the walls of the spore may be stained. This stage is seen in Fig. 42. The 
cytoplasm outside the spore stains only very slightly at this stage, because 
doubtless most of it has been absorbed by the young spore. This stage is 
best observed in a three days old culture. In such a culture it is usual to 
see actively motile individuals in which mature spores are enclosed. The 
cytoplasm, therefore, cannot all have been absorbed into the spore in such 
cases, for there must be some left to maintain communication with the 
cilia, and it is obvious that when all the cytoplasm has been absorbed into 
the spore, the cilia must drop off and motion cease. This fifth stage is 
represented in Fig. 43, which shows a mature spore resting inside a perfectly 
empty sac. Nothing remains except the bare cell-wall. How long the 
cell-wall can remain intact without disruption must depend on the medium, 
and more especially on the excreted products. If the latter were acid in 
reaction, the cell-wall would soon be dissolved. Whatever the dissolving 
