Fig. 18. — ^Transverse sections through young embryos of Amphioxus, showmg 

 formation of nerve cord, notochord, and mesoderm. A. Commencement of the 

 growth of the superficial ectoderm (neural folds) above the neural plate (medullary 

 plate). B. Continued growth of the ectoderm over the neural plate. Differentia- 

 tion of the notochord, and first indications of mesoderm and enterocoeUc cavities. 

 C. Section through middle of larva with two somites. Neural plate folding 

 into 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 fibriUse are forming, ar, archenteron; c, enterocoel; 

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

 unsegmented mesoderm fold; ms, mesodermal somite; nc, neurocoBl; nf, neural 

 fold; np, neural plate; nl, neural tube. (From Kellicott's, " Outlines of Chordate 

 Development " [Henry Holt and Company] after Cerfontaine.) 



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