Evidence from Somatic Histogenesis 61 



So we come again to the real issue. Assuming the Z 

 membrane to be a cytoplasmic structure, as it has practi- 

 cally always been held to be, are we going to deny that the 

 cytoplasm itself causes the peculiarity of the Z membrane 

 in tlie sea-spider as compared with that in Limidus, that 

 denial being necessitated by the dogma tliat the real "seat" 

 of the difference is the chromatin of the nucleus operating 

 by some invisible "factor" perhaps of the nature of an 

 enzyme? 



The extent of variety in striated muscle tissue is brouglit 

 impressively to view in such a comparative study as that 

 by Marceau. His main object is not to find differences but 

 to discover whether in spite of structural differences they 

 have similar traits, as if they might all be derived from a 

 single primitive form which has undergone more or less 

 profound modification. 



Of the many differences which the investigation sought to re- 

 duce to orderliness on the basis indicated, only two will be men- 

 tioned. From an elaborate table of measurements of the diameter 

 of fibers, we find the following results for the sheep and pig: "^ 



Maximum Minimum Average 



Sheep 25 fi Sfx 15ju 



Pig 45 5 20 



The other point selected concerns "striated scleriform trans- 

 verse bands" characteristic of the muscle fibers of the vertehrate 

 heart. This time the animals we choose are the liorse and cow. 

 The thickness of the band is jjiven as exactlv the same in thesr 

 two, but the distance between the bands is 1 lO^u for tlie horse and 

 120/1 for the cow.^* 



The FliysioJogy of Miiscle Fibers 



We might go on almost endlessly, pointing out slight hut con- 

 stant specific differences that involve differences in muscle struc- 

 ture, questioning in each case whether the hereditary cause of 

 this difference lies in the cytoplasm or chromatin of the nmscle 



