M. E. MacGregor 
187 
specimens. It consists, as will be seen in fig. 12, of rods that are 
thickened at their centre in such a way that a diamond or square-shaped 
formation results, which is hyaline to some extent and highly refractive, 
so much so that at the high magnifications that have to be used when 
examining this structure, it has not been by any means easy to feel 
certain when its true focus had been attained. 
The base on which these sinuous “ slits ” are placed, and which has 
been referred to as the “ plate,” when examined from the exterior 
surface presents the appearance described above, i.e. an appearance 
suggesting its possible formation by the fusion of the two separate 
plates found in the schizotreme- and ptychotreme-types of stigmata : 
examined from the under surface of the section however, a strikingly 
beautiful and remarkable structure is seen. Observed in this position, 
there can be seen, arising at regular intervals on the lower surface of 
the plate, six to eight chitinous processes, which curving upwards fuse 
with a small ring directly above the centre at the back of the plate, and 
thus form, as it were, a gourd-shaped basket. I was fortunate enough 
to be able to obtain a fairly large number of these larvae alive from the 
stomach of a horse that was autopsied at the Vaccine Laboratory here, 
and was in this way permitted to study the stigmata in much greater 
detail than has been the case with many of the other specimens, in that 
the physiology as well as the anatomy could receive attention. It 
would be beyond the purpose of this paper to discuss the former subject 
here, but it might be interesting to state that the physiology of these 
organs appears by no means as simple as it has been supposed, and that 
while the sinuous bar systems, or so-called “ slits,” have been described 
as lying on the plate, they are in reality attached to a very delicate 
transparent membrane which covers it, and in the fresh larvae it has 
been found possible, under the microscope and using special instruments, 
to dissect off this membrane, bringing away with it the bar systems 
intact (see fig. 11). Breadth of stigmal plate, 2'6-3‘l mm. 
Oestris ovis Linn. (Plate XVI, figs. 13 and 14.) 
Here again the stigmata are found to be two separate plates, but 
the conspicuous “ ring ” is entirely absent. These plates are from 
1'20-1'26 mm. in breadth, and the distance from “ button ” to “ button ” 
is from 1 ’24-1'30 mm. The “buttons” are very well defined, and 
from them there radiates over the entire surface of the plates a very 
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