D. Keilin 
101 
--S.Z. 
succeeds in surrounding them in the form of a ring. The three surrounded cells 
change their shape, becoming flattened, in the form of three superposed or 
parallel discs which occupy the centre of the ring formed by the fourth cell 
(Figs. 29 and 30). This cell then secretes an external membrane which enve¬ 
lopes the whole group of four cells, thus forming a spore. 
We now arrive at a stage in which the spore is most frequently encountered 
and which is easily recognised. It is the barrel-shaped spore surrounded by 
a very fine and transparent membrane or sporocyst. Its largest diameter is 
between 5 and 6/x. When examined end-wise, the spore shows a central 
circular mass, strongly basophile, and surrounded by a highly refractive ring 
which fills the space between the central mass and the walls of the sporocyst 
(PI. IV, fig. 31). 
Examined from the side, the spore shows that the deeply-stained central 
mass is composed of three superposed discs, parallel to each other and to the 
flattened surfaces of the barrel-shaped spore (PI. IV, fig. 32). These central 
discs are surrounded by a refractive ring which we now see from the side only. 
Only by careful differentiation can the vesicular nuclei 
be detected in the central discoidal cells. These spores 
now undergo a further transformation: examined in vivo 
they show a spiral refringent band surrounding the three 
central cells and lining the sporocyst. The more detailed • 
structure of this stage can be seen only in fixed and stained 
smears or in sections of the infested larva. 
The protoplasm of the three central cells now loses Text -fig- !• Completely 
more and more its basophile property, while m the re- the three central cells 
fringent ring which surrounds these cells a chromatic 0 r sporozoites—*.z.; a 
substance appears which assumes the form of a spiral peripheral spiral fila- 
band with 3-4 turns surrounding the central cells of the ment— /• * and the ex ' 
/r _ „ , n ^ ^ ternal membrane or 
spore (Text-fig. 1 and PL IV, figs. 33, 34, 35, 36 and 37). sp0 rocyst —a.p.. 
Viewed from the polar ends of the spore, the chromatic 
spiral band appears as a series of superposed chromatic rings surrounding the 
central cells (PL IV, figs. 38-42). During differentiation after staining with 
iron-haematoxylin, the chromatic spiral still retains a very dark colour, after 
all the rest of the spore is completely decolorised. 
At this stage the nuclei of the central cells are distinct; the nuclei are 
variable in shape but are usually discoidal, their chromatin forming a peri¬ 
pheral ring which is connected with a central body or karyosome of an irregular 
form. 
The parasite now invades the whole body of its host to such an extent that 
the latter dies, and as we have seen, the host’s tissues are destroyed and 
replaced by a solid mass composed solely of these spores. For a long time 
I supposed that these spores represented the final developmental stage of the 
parasite, namely a resistant form, which, being set free from the dead host, 
were swallowed by other larvae of Dasyhelea which thus became infected. 
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