No. I.] MESENCHYME IN CERTAIN LARVAE. 



19 



been observed in " single fibrils in protoplasm, as well as con- 

 tractile pellicles and substance membranes," and also in Meta- 

 zoan cilia and the muscle bands in rotifers. ^ It seems to be 

 carried farther here than in the muscles mentioned, for the 

 simple reason that these filaments are unattached and, there- 

 fore, there is nothing to restrain it. 



After remaining an instant in this extreme contraction, the 

 cells relax, and the return to a normal condition is practically 

 instantaneous. We have here, then, the same power manifested 

 by the single mesenchyme cell, with its branches, that belongs 

 to the more complicated retractor muscle, and that, too, when 

 it is isolated from everything save the liquid in which it floats. 



This must certainly be a very near approach to a primitive 

 muscular contractility. 



The contraction lasts one and a half or two seconds, the 

 relaxation occupies but a very small fraction of a second, while 

 the pause or rest varies from two or three to twelve or fifteen 

 seconds. This suggests very forcibly a condition similar to 

 that which obtains in the beating of the heart, with the 

 exception that in these mesenchyme cells the relative duration 

 of contraction and relaxation is reversed, the former beingf 

 much longer. 



This same contractile power is also possessed, to a less 

 degree, by the other mesenchyme cells. They may often be 

 seen to contract after they have become branched. The con- 

 tractions are not as rhythmical as those just described, but 

 they are as automatic. Some of these mesenchyme cells enter 

 the velum during development and become attached to its walls 

 until the whole interior is traversed from wall to wall by their 

 branches, rendering it highly contractile.^ 



In this case, therefore, the same cell which contracts at first 

 automatically may afterward become a part of the muscular 

 network of the velum, where it is under the control of the 

 central nervous system. 



Similar phenomena were observed in the mesenchyme cells 

 of the pilidium larvae of the Nemertean Ccrebratiilus lacteiis 



1 Andrews, G. F., The Living Substance as Such: and as Organism, p. 103. 

 1897. ^ Lang, Comparative Anatomy, Part II, p. 257. 



