Brown.—Studies in the Physiology of Parasitism. /. 325 
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The limitations of this method, and the accuracy of which it is capable, 
are discussed under the following five headings: 
(i) Nature of tissue employed. It is obvious that a tissue suitable for 
this purpose is one which in its fresh state possesses a marked degree of 
coherence. Such a tissue, as for instance that of the pear or even of many 
varieties of apple, is of little value; on the other hand, turnip and potato 
tissues are eminently adapted to the present purpose, and these have 
accordingly been generally employed. 
The ideal tissue is one which allows of discs being cut which are 
uniform in quality throughout. Ordinary leaf tissue cannot therefore be 
employed, and in practice we are confined to the use of fleshy tissues. 
Even with these it is in general impossible to prepare discs which are quite 
uniform throughout. Thus even in the centre of the potato small vascular 
strands occur which produce local lack of uniformity. In cases where these 
are abundant, the tissue should be rejected, and in all cases care must be 
taken that these small vascular strands do not run in the plane of the 
section and more especially in the direction in which coherence is tested. 
A tissue suitable for the present purpose may therefore be defined as one 
furnishing discs which on microscopical examination are seen to consist of 
a ground-mass of uniform parenchyma cells with a limited number of islands 
of vascular (more resistant) tissue. 
(ii) Accuracy in thickness of the sections employed. The degree of 
accuracy obtainable in the disc-cutting process depends to a considerable 
extent upon the nature and physiological condition of the particular 
tissue. It also depends upon the thinness of the discs cut, there being 
a limit of thinness beyond which the microtome ceases to perform satis¬ 
factorily in this respect. Care must be taken that the cylinder of tissue is 
sufficiently rigid not to yield to the knife. This consideration requires that 
the cylinder, in addition to being of a certain stoutness, should be quite 
turgid, a condition which is readily obtained by injection with water 
previous to cutting. In the case of potato tissue, the upper limit of 
variation in thickness of the microtomed discs was determined as 
4 per cent. 1 
(iii) The determination of the ‘ end-point 3 of reaction. It must 
be conceded that the arbitrarily chosen end-point does not represent 
any definite stage of the reaction. It is simply that stage at which 
1 This was measured by cutting a series of sections, which were then washed, rapidly dried 
between filter-papers, and weighed. The following were the figures obtained : 
Max. weighing ..... .... o*o88 gr. 
Min. „.0*085 g r * 
Average weight of sixteen discs.0*0867 gr. 
Max. variation from mean. ...... 0*0017 gr. 
Max. variation among the individual readings . . . 0*003 gr. 
This variation of 0*003 g r * in 0*0867 gr. is approximately 4 per cent. 
