178 CHAS. B. WILSON. 



them are longer than in later development, but they do not 

 stop here. 



5. The muscle-fibres thus formed increase both 

 in size and contractile power through exercise. 



When first formed the fibres are simply fibrillated pseudo- 

 podia of single mesenchyme cells, hence all their substance 

 must come from this one cell. And yet there is a perceptible 

 increase in the size of the fibres, even after the entire body 

 of the cell has gone out into them. This increase is most 

 rapid and hence most apparent in those muscles which are 

 most often contracted, e. g. the apical and circumoral 

 muscles. 



The increase in strength is admirably shown in the lappet 

 muscles, each of whose single fibres scarcely shortens at all 

 in its initial contractions. But after several days' develop- 

 ment it can contract so vigorously as not merely to shorten, 

 but also to assume the characteristic corkscrew shape. 



6. Fibrillation and the consequent change from 

 potential to kinetic contractility occurs only after 

 the pseudopodium has attached itself to something 

 outside the cell. 



We found the same thing true of the spin-threads sent out 

 by the polar bodies and the segmenting blastomeres. 



The simplest explanation of this fact is that the constant 

 movement of the cytoplasm in pushing out the spin-thread 

 prevents contraction, but as soon as spin-thread or pseudo- 

 podium becomes attached the flow ceases, fibrillse appear, and 

 contraction ensues. 



It is worthy of note that the nature of the contractions 

 depends somewhat on the kind of attachment. When 

 attached to a cell of the pilidium wall the contractions in 

 the resultant muscle-fibre are like those of ordinary muscle. 

 When threads from two cells anastomose without becoming 

 attached to anything else, they contract in rhythmic pulsa- 

 tions like cardiac muscle. 



This rhythm is shown even better in the muscles formed 

 from mesenchyme cells in the veliger larvse of certain nudi- 



