T11E INKER TISSUES OF ANIMALS. 355 



the primordial muscular substance ? or has the frequent recur 

 mice of irritations and consequent contractions at particular 

 parts done it ? We have, I think, reason to conclude that 

 direct equilibration rather than indirect equilibration has been 

 chiefly operative. The reasoning that was used in the case 

 of nerve applies equally in the case of muscle. A portion of 

 uridifFerentiated tissue containing a predominance of the colloid 

 that contracts in changing, will, during each change, tend 

 to form new molecules of its own type from the other colloids 

 diffused through it : the tendency of these entangled colloids 

 to fall into unity with those around them, will be aided by 

 every shock of isomeric transformation. Hence, repeated 

 contractions will further the growth of the contracting mass, 

 and advance its differentiation and. integration. If, 



too, we remember that the muscular colloid is made to 

 contract by mechanical disturbance, and that among me- 

 chanical disturbances one which will most readily affect it 

 simultaneously throughout its mass is caused by stretching, 

 we shall be considerably helped towards understanding how 

 the contractile tissues are developed. If extension of a mus- 

 cular colloid previously at rest, produces in it that molecular 

 disturbance that leads to isomeric change and decrease of 

 bulk, then there is no difficulty in explaining the movements 

 of cilia. The formation of a contractile layer in the vascular 

 system becomes comprehensible : each dilatation of a blood- 

 vessel caused by a gush of blood, will be followed by a con- 

 striction ; the heart will pulsate violently in proportion as 

 it is violently distended ; arteries will develop in power as 

 the stress upon them becomes greater. And we shall simi- 

 larly have an explanation of the increased muscularity of 

 the alimentary canal that is brought about by increased 

 distension of it. 



That the production of contractile tissue in certain localities, 

 is due to the more frequent excitement in those localities 

 of the contractility possessed by undifferentiated tissue in 

 general, is a view harmonizing with facts which the diffe- 



