252 PRIMARY FACTORS OF ORGANIC EVOLUTION. 



on to describe the effect of such fixation of joints on 

 the bones themselves, to which I will refer on a later 

 page, under the head of the origin of articular sur- 

 faces. 



Professor Eimer of Tubingen has given us a synop- 

 sis of the nature of the evolution of the characters of 

 the muscular tissue, which is highly instructive, and 

 of which I present here an abstract.^ The conclusions 

 reached by Eimer are derived from a general study of 

 the subject, both in the laboratory and in the litera- 

 ture. He says : 



" (i) It is apparently continued contractions of the 

 protoplasm in definite directions, which have produced 

 muscular masses. Since plants do not display con- 

 tinuous and vigorous movements, they have not de- 

 veloped muscular bodies. 



"(2) Undoubted facts indicate that from a primi- 

 tively identical substance muscular tissue has devel- 

 oped in the direction of effective contractions, while 

 connective tissue has developed where no contractions 

 have been present. 



"(3) Muscle-masses first appeared almost every- 

 where in the external layer of the contractile region : 



" «, in Protozoa in the outer layer of the body; 



'■'■ b, in Metazoa in the tegumentary sheath of 

 the body. 



*V, They consist first either of muscle-cells, or 

 muscle-fibers, from which develop man- 

 tle muscle-cells and mantle muscle-fibers. 

 Mantle muscle-iibers compose the other- 

 wise highly developed striped muscles of 



1 " Dio Entstehung und Ausbildung des Muskelgewebes, insbesondere 

 der (juerstreifung desselben als Wirkung der Thatigkeit betrachtet." Zeit- 

 schrijt fUr wissenschafiliche Zoologie, LIII., Suppl., 1892, p. 67. 



