ii 4 THE AMPHIOXUS. 



served as well in the optical anterior sections (Fig. 52), 

 which we get on a dorsal view of the embryo, as on 

 the lateral view (Fig. 51). 



These elongated cells in contact with the notochord, 

 which later on provide the muscular system, are found 

 in definite numbers and in regular arrangement. This 

 is the case in so far as they are arranged lengthwise 







in regular rows, so that the individual cells of the 

 primitive segments, which follow one another, are 

 in contact with each other. 



We can, too, on a lateral view of the stage with 

 nine primitive segments, recognise a further advance 

 in the posterior undifferentiated region of the body, 

 while, too, the undifferentiated mesoblast fold experi- 

 ences a distinct lateral separation. 



In the whole embryos another and important ap- 

 pearance may be followed, which concerns the first 

 mesoblastic somite. This is the formation of a hollow 

 continuation from it, which grows gradually forward, 

 right into the end of the body, and breaks through that 

 region of the body in which no mesoblast was formed. 



Let us now consider, on transverse sections, the 

 changes which the mesoblastic somites experience in 

 this period of development. 



