144 



intrafihrillar structure, they arc much surpassed by the brilliant iron haematoxylin 

 method of staining. For the examination of entire fibres ordinary glycerine or 

 gold chloride mounts are sufficient. 



Schafer's observations on the minute structure of the fibrils were mainly 

 made with the aid of the sarcostyles of insect wing muscles. A much more 

 suitable material to employ, however, is the muscular tissue from fully-grown 

 insect larvae. Growth of the larvae of metabolic insects takes place mainly as 

 a result of a great hypertrophy of cells which are present already in newly-hatched 

 larvae ; there is no increase in their number, but they merely grow in size. 

 There is usually not even an increase of their differentiation ; everything, cellular 

 and intracellular, is merely greatly hypertrophied, and such tissues obviously 

 offer a great advantage for the study of structures which are just visible with 

 the highest powers of the microscope. This will be clearly realized by comparing 

 figs. 3 and 7 (pis. xi. and xii.) ; both are magnified 3,400 times. Fig. 3 is a 

 fibril from a nearly full-grown beetle larva ; fig. 7 represents some sarcostyles 

 from the wing muscles of a wasp. 



The results obtained with this excellent material I have throughout verified 

 by the examination of skeletal muscle fibres from vertebrates, and the leg 

 muscles of adult insects. 



A remark may be made here regarding the interpretation to be placed on 

 high-power examination of stained preparations. Many regard structures 

 visible under such magnifications as artefacts due to the reagents employed. 

 While this cannot be denied, it may be pointed out that if there is any dis- 

 tortion, some structures must have been previously present to be distorted ; 

 and while we cannot ascertain the actual living appearance of many of the 

 minute structures thus rendered visible, still we can usually be certain of their 

 definite existence. It should also be pointed out that results obtained by the 

 high-power examinations of mitotic figures have been largely verified by the 

 examination of living material. 



In a recent paper (1922) I pointed out that the striations of voluntary 

 muscle fibres did not run transversely across the fibres in the form of discs, as 

 Bowman had originally supposed, but that they were disposed in the form of 

 a double spiral which travelled from one end of the muscle fibre to the other. 

 A double helicoid would have been a more accurate description. This structure 

 is not, of course, to be looked upon as a continuous membrane in the form of 

 a double helicoid, but is a structure composed of minute sarcous elements packed 

 closely together, and so disposed within the fibre as to form the double helicoid. 

 Usually this spiral arrangement of the striations is most difficult to detect, on 

 account of the closeness with which it is w^ound. The observation becomes 

 much easier if well-stretched fibres are examined, but even then considerable 

 care is needed to observe it. It was the helicoid which Leeuwhenhoek saw and 

 which he regarded as a thread running spirally around the fibre. It was also 

 a partial view of this structure that Schafer observed when he wrote : "When 

 the muscular fibres are deeply focussed, the appearance of the striae becomes 

 somewhat altered, and a fine line, often dotted, is seen passing across the 

 middle of each light band." This is regarded as constituting Dobie's line, but 

 is not identical with the structure spoken of by that name in the historical 

 introduction above. The line is visible only in the middle of the fibre, and the 

 smallest turn of the focussing screw downwards shows it to be continuous with 

 another striation on the "lower" half of the fibre. It is. indeed, merely the 

 indistinct view of the turn of the spiral on the opposite side, and not a distinct 

 structure. 



A careful examination of hypertrophied larval insect muscles amply justifies 

 the view held by Schafer that Krause's membrane is a complete membrane, passing 



