BLASTULA, GASTRULA, AND GERM LAYERS 337 



np nf 



D 



FIG. 152. Transverse sections through young embryos of Amphioxus, showing 

 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 ectoderm over 

 the neural plate. Differentiation of the notochord, and first indications of meso- 

 derm and enterocoelic 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 fibrillae are form- 

 ing (compare Fig. 153). ar, archenteron; c, enteroccel; ch, notochord; ec, 

 ectoderm; en, endoderm;/, muscle fibrillae; g, gut cavity; ra, mesoderm (gastral); 

 ms, mesodermal somite; nc, neural canal; nf, neural fold; np, neural plate (med- 

 ullary plate) ; nt, neural tube. 



