B. T. LOWNE ON THE HISTOLOGY OP THE MUSCLES OF THE FLY. 183 



the intermediary substance between the columns forming the line 

 or boundaries of Cohnheim's fields is muscle plasma, and is 

 continuous with the nucleated layer immediately within the sarco- 

 lemma. 



I regard the muscle fibres of the heart as colonnettes, the trans- 

 verse section of a single fibre representing a large field of 

 Cohnheim. 



The above view of a muscle fibre appears to be borne out by 

 a comparative study of the three forms of striated muscle fibre 

 which exist in the blow-fly and other insects, and I should describe 

 a striated muscle fibre as consisting of one or more columns of 

 primitive fibrillar cemented together, forming one or more colon- 

 nettes. In the ordinary mammalian muscle fibre a number of 

 colonnettes are embedded in a nucleated plasma, and surrounded by 

 a muscle-sheath. 



All the varieties of striated muscle in the fly possess the above 

 characters, but it is only in the larva that the fibres correspond with 

 those of the mammalia, in which numerous colonnettes are em- 

 bedded in plasma, and surrounded by a membranous sheath, sarco- 

 lemma, with the nuclei of the plasma immediately beneath it. 



It is noteworthy that the muscles which we might be tempted to 

 regard as the most highly differentiated form of contractile tissue 

 exist only in the insect in its larval condition, when development is 

 but little advanced above the vermiform type. 



Physiologically, the muscles of the larva of the fly have, how- 

 ever, the same functions as in the higher vertebrata ; they con- 

 tract powerfully, and the duration of the contraction is compara- 

 tively long ; fatigue does not supervene rapidly. Such muscles 

 respond to a second stimulation immediately after contraction, and 

 the contraction is tetanic, not clonic. 



With regard to the ultimate structure of the colonnettes, or 

 bundles of ultimate fibrillar, the optical appearances differ much, 

 according to the method of illumination and the kind of objective 

 used. It is easy to make out alternate dark and light bands. With 

 my immersion ^g, the bright bands, in specimens fixed with osmic 

 acid, appear very narrow and well defined, and the dark bands 

 appear to consist of dark prisms, separated from each other by very 

 fine bright lines running in the direction of the fibre ; but with a 

 quarter, and the same illumination, the dark and light transverse 

 bands change places with each other, the bright bands becoming 



