134 THE AMPHIOXUS. 



consist of columnar cells, whose behaviour we have 

 previously described in detail. 



The differentiation of the muscles has now its 

 origin in these cells, more or less in the stages with 

 ten mesoblastic somites. In the larvae stages with 

 eleven mesoblastic somites I could already observe 

 slight lateral shrinkage which is to be referred to 

 the action of the muscles. The fibrilla, which at 

 first were very faint, became continually more 

 distinct in the course of this period of development. 



It may be seen that each cell forms at first only 

 a single fibrillum, moreover the muscle cells close 

 together in longitudinal series and a segmental 

 interruption is not to be seen in the separate fibrilla. 

 It may therefore really be said that a cell now forms 

 a common fibrillam which may be followed straight 

 on through the length of the body, and that each 

 separate cell becomes a segment in the formation of 

 such a long fibrillurn. 



There may be seen transverse stripes of the fibres 

 both in the living object and in preparations, this 

 being especially the case when they become more 

 distinctly prominent. 



In order to become acquainted with the manner 



