60 The Unity of the Organism 



hereditary attributes determined by the chromatin or by 

 the cytoplasm of the myogenic cells? Applying our usual 

 test for hereditary structures, we ask whether or not this 

 tissue presents attributes characteristic of the taxonomic 

 groups, species, genera, families and so on. Here again, 

 while we have a vast store of knowledge about the structure 

 and genesis of muscle tissue in many kinds of animals, only 

 incidentally, as a rule, have the studies been made from the 

 taxonomic standpoint. 



The Morphology of Striated Muscle Fibers 



For one series of very recent studies that comes near this 

 standpoint w r e are indebted to H. E. Jordan. On the taxo- 

 nomic aspect of the matter Jordan writes : "A quite gen- 

 eral consensus of opinion considers them more or less closely 

 related, and ranks them both between Crustaceans and 

 Arachnoids. Limulus muscle, however, is in appearance very 

 much more like vertebrate than like insect muscle ; while the 

 muscle of the marine arthropod Anoplodactylus is of the 

 typical insect type." 21 Of the numerous differences between 

 the fibers of the two animals compared, reference to two will 

 suffice for our purpose. They concern the so-called M 

 and Z lines found in the light bands of most striated muscle 

 tissue. What Jordan regards as one of the important re- 

 sults of his work on Limulus muscle was the evidence secured 

 that the Z line represents a membrane, as some observers 

 have believed, the specially convincing evidence being the 

 fact that the "line" is attached to the sarcolemma periph- 

 erally and to the nuclear wall centrally. These rela- 

 tions are lacking, he says, in the sea-spider's muscle. The 

 M line, he tells us, is especially well developed and hence 

 easily demonstrated in the sea-spider muscle in some states 

 of contraction, while he failed, as have other students, to 

 detect it at all in the Limulus muscle. 22 



