475 



that I have examined muscles in both fore and hind extremities of the 

 white rat and got a decrease. In the field vole the percentage of 

 fibres in the triceps of young from nest and of adult was 100:46, 

 in the case of the sterno - mastoid it was 100:81. I may take this 

 opportunity of correcting a mistake I fell into in my previous paper 

 — a mistake in the naming of the species. The field vole (Arvi- 

 cola agrestis) is usually called in the country here the field mouse 

 and when the specimens were brought in they were labelled accordingly. 

 In removing the muscles, I did not notice the mistake and hence I 

 described the muscles as belonging to the field mouse. 



The biceps of the cat, at 9 days, 20 days, 240 days, and over 

 3 years gave the following percentages 100:77:45:27. One of the 

 heads of the perforans of the sheep gave in the late embryo 42°/ , 

 in a 2 — 3 weeks old example 100 °/ and in a 3 months old stage 

 46 Vo- 



Other calculations made on other individuals show conclusively 

 the reduction which occurs in the number of fibres. They illustrate 

 moreover that females have a less number of fibres than males, at 

 the same stage and that while the two sides of the body are fairly 

 equal in the young condition variation becomes more marked with age. 



I have in several cases counted every fibre and in others made 

 elaborate calculations. These I have repeated on several sections be- 

 longing to the same series and with different powers and I am there- 

 fore confident of my results. 



It may not then be out of place to briefly contrast Moepurgo's 

 method with the one I have adopted. Like Morpurgo I began my 

 attempts at enumeration by making a drawing, but my drawing dif- 

 fered from his in that I used an eye-piece micrometer ruled in squares 

 and filled in the drawing on paper similarly ruled in squares. But I 

 soon abandoned this laborious method, and adopted the plan I de- 

 scribed in my preliminary communication. This was, simply to fill into 

 the squares the number of fibres actually counted and to add up the 

 totals. This direct method seems less liable to error than drawing 

 the section with a camera lucida, and then counting. 



The muscle spindles are not to be looked upon as the result of 

 the process of reduction. Indeed they appear to share in the degene- 

 ration, for the intrafusal fibres are more numerous in the foetus than 

 in the adult (Felix) *). Besides they are not numerous enough to 



1) Felix, Zeitschr. f. wiss. Zool., Bd. 48, 1889, p. 224. 



