THE DEVELOPMENT OF AMPHIOXUS 



21 



np 



FIG. S. Transverse sections through young embryos of Amphioxus, show- 

 ing formation of nerve cord, notochord, and mesoderm. After Cerfontaine. 

 A. Commencement of the growth of the superficial ectoderm (neural folds) 

 above the neural plate (medullary plate). B. Continued growth of the ecto- 

 derm over the neural plate. Differentiation of the notochord, and first indica- 

 tions of mesoderm and enteroccelic cavities. C. Section through middle of 

 larva with two somites. Neural plate folding into a tube. D. Section through 

 first pair of mesodermal somites, now completely constricted off. E. Section 

 through middle of larva with nine pairs of somites. Neural plate folded into a 

 tube. Notochord completely separated. In the inner cells of the somites, 

 muscle fibrillse are forming (compare Fig. 10). ar, archenteron; c, enteroccel; 

 ch, notochord; ec, ectoderm; en, endoderm;/, muscle fibrillse; g, gut cavity; m, 

 unsegmented mesoderm fold; rns, mesodermal somite; nc, neurocoel; nf, neural 

 fold; up; neural plate; nt, neural tube. 



