FOURTH PERIOD OF DEVELOPMENT. 135 



in which the fibres become separated from the cells 

 we will take into consideration single preparations as 

 well as sections. 



In Fig. 59, we see isolated a part of the notochord 

 with the muscle cells attached. It may there be 

 seen that -the fibrilla are formed in contact with the 

 median side of the cells, i.e. they are immediately 

 in contact with the notochord. On a surface view 

 of the muscle band (Fig. 58) we may with one 

 position of the microscope see the elongated proto- 

 plasmic bodies of the cells which even yet contain 

 single yolk granules. In the places where the cell 

 granules lie the cells are swollen in a spindle-like 

 shape. It may further be here seen how the cells 

 of contiguous mesoblastic somites are directly con- 

 tinuous the one into the other. On lowering the 

 microscope the small elongated muscle fibrilla are 

 seen whose number agrees with that of the cells. 



With reference to this agreement we are readily 

 liable to mistakes in single preparations, since a 

 number of the cell granules may be easily torn away 

 from the muscle band, while the fibrilla belonging 

 to it lie before us. Owing also to disarrangement 

 of the contours a larger number may be indistinctly 



