92 THP: royal society of CANADA 



present practically from the very first and is not a secondary develop- 

 ment, as Godlewski^ describes. Very soon each pair of dots becomes 

 converted into a dumbell shaped structure, owing to the intervening 

 clear zone assuming slight staining properties, as shown in Fig. 7. 

 Each primitive muscle fibril at this stage may thus be regarded as 

 built up of a series of dumbell shaped structures whose long axes 

 are placed at regular intervals in a longitudinal direction. It will 

 be already evident that these "are the sarcous elements of Bowman 

 for each individual fibril. When placed alongside those of neighbour- 

 ing fibrils a clear line or space shows across each apposed series of 

 dumbells, corresponding to the constricted portions of the latter 

 and obviously represents the line of Hensen. The structure of Dobie's 

 line likewise becomes easily analysed from a study of its histogenesis. 

 In each fibril a minute» dot of chromatic material makes its appear- 

 ance in the centres of the clear spaces between the elements of Bowman 

 and at a slightly later period of development (Fig. 1). A similar 

 dotted arrangement appears simultaneously in neighbouring fibrils, 

 the result being of course the production of Dobie's line. 



It should be specially noted at this point that the striation does 

 not at first involve the whole thickness of the muscle fibre, but is 

 strictly limited to the individual fibrils. A striped muscle fibre is 

 thus the sum total of the striations affecting or involving its constituent 

 fibrils. It will therefore be at once recognised that the histogenetic 

 unit in striated muscle is not the fibre but the fibril. 



The developing muscle fibrils always appear in definite groups 

 or bundles which constitute the foreshadowings of the muscle fibres. 

 This fact is shown in several of the microphotographs; but is most 

 strikingly evident in the transverse sections (Figs. 11, 12, 13 and 14). 

 The exact relationship of these to the nuclei is difficult to determine 

 with any degree of accuracy, but in some species (Zoarces, Lacerta 

 and Canis for example) those portions of the myo-syncytium farthest 

 away from the nuclei become fibrillated first of all. The result is 

 that in these species the first formed fibrils arrange themselves in 

 circles which form the peripheral portions of the developing muscle 

 fibres and enclose the nuclei (Fig. 11). The fibres are thus very 

 early mapped out in these species in the form of ring shaped outlines 

 which present a very characteristic appearance in transverse sections 

 of the developing muscle tissue. 



iQp. cit. 



