620 



on water on an albumenised slide. In this manner every fibre was 

 rendered quite distinct. Many other stains were tried including several 

 of the well-known double stains but for the purpose in view well- 

 stained carmine sections left nothing to be desired.' 



As fairly comparative a section as possible was chosen in each 

 case and the fibres counted by means of an eye-piece micrometer ruled 

 in squares. A paper similarly ruled in squares was filled in with the 

 numbers as the fibres were counted. 



Average per Area of 



Triceps of ^^^- square of muscle ^J"^^®^ 



of fibres Q^5;^ ^^ ^^^^.^^ of fibre 



young Field Mouse . ^ ^„ ,, ... ^ ^k a nnno 



from nest 10 070 114 1.95 mm 0.0002 mm 



adult 4 613 17 4.96 mm 0.0013 mm 



The above table requires little explanation : The area of the adult 

 fibre in section is nearly seven times that of the young. It includes 

 of course its share of the space in which it lies. The area of the 

 muscle section was obtained by counting the number of squares of 

 known area covered by the section. The area of the fibre was got 

 by dividing the average number per square into the area of the 

 square. These calculations, moreover, made with a different objective 

 to that used for the enumeration will be found to be very confirm- 

 atory of the numbers in the first column. 



A similar examination of 2) the biceps of the Cat at dif- 

 ferent ages was made with the following results: 



^^ Average ^ r- il r 



n-^^^c ^4^ -^0- ^ cr. ^f Area oi Area oi 



Biceps ol „ „, p. sq. ot 



or nbres 0263 muscle nbre 



Cat 9 days old 83 514 7 8.4 0.0001 



„ 20 „ „ 64108 5.5 8.1 0.00013 



„ 240 „ „ 37 830 1.15 22.8 0.0006 



These cats all belonged to the same litter. The numbers of 

 fibres were, however, not got by enumeration but by calculation. 

 In the first two stages a portion was counted with a Leitz objective 7. 

 Then by using different lower powers the number of squares covered 

 in each case was noted. The number of squares with objective 7 

 contained in each of the other powers was determined by actual 

 measurement with a micrometer slide; and thus the approximate 



