326 ORGANIC GROWTH 



ORIGIN OF MUSCLES 



A striking proof of the importance of activity, function, in 

 the evolution of the organism is afforded by the mesoderm in 

 the muscles of that layer. 



In unicellular forms and the larvae of the multicellular 

 free and rapid movement in the water is effected by flagella. 

 Muscles evidently first arise in consequence of ever-repeated 

 changes of form, of the bending, extension, and contraction of 

 the body, by which it is diminished in order to avoid danger, 

 or enlarged in order to seize food, or moved in various ways 

 in fighting and in locomotion. 



It is well known to histologists how difficult it sometimes 

 is to distinguish smooth muscle-cells from certain connective- 

 tissue-cells. Both arise in the mesoderm, from the same 

 fundamental layer. I showed in a particular case years ago 

 that, in fact, connective-tissue and muscle pass directly one 

 into the other, the latter developing first in those directions 

 of the body in which contraction takes place, while in the 

 rest only connective tissue arises. This is the case among 

 the Ctenophora in Beroe ovatus, where the gelatinous tissue 

 of the body is traversed by connective-tissue-fibres and very 

 simple muscle-fibres, which pass into one another so gradually 

 that a boundary between the two cannot, in spite of all 

 endeavour, be determined. The muscle-fibres consist of long 

 tubes of contractile substance, which in the fresh condition 

 seem homogeneous, but can be separated into longitudinal 

 fibrils by reagents, and which contain unorganised protoplasm 

 containing nuclei at intervals. These fibres are surrounded 

 by a sarcolemma. " The typical connective-tissue-fibres are 

 cylindrical, strongly refracting, delicate threads running a 

 straight or contorted course, and are usually enlarged to a 

 spindle-shape at places by nuclei, which, however, lie at 

 very long intervals from each other. In the coarser fibres a 



