THE CEPHALOCORDA 



195 



manner from the archenteric cavity, it is usual to call it an 

 enterocoele. 



The second pair of ccelomic cavities is formed as a pair of 

 outgrowths of the archenteron in front of and rather above the 



Fig. 49 



A,B,C. Transverse sections of embryos of Amphioxus of different ages, 

 illustrating the formation of the neural tube, notochord, and mesoblastic 

 somites. a.ch^ archenteron ; mes, mesoblastic somites ; nch, notochord ; 

 m/, neural plate ; nt, neural tube. D. Transverse section through an 

 older embryo in which the mesoblastic somites are completely separated 

 from the archenteron. jnp, muscle plate ; cce, coelom; en, mesenteron ; 

 the other lettering as above. E. Diagrammatic transverse section 

 through a larva with five gill-sUts. vtyc, myocosle ; spl, splanchnoccele. 

 F. Diagrammatic transverse section of a young Amphioxus shortly 

 after the metamorphosis ; the section is taken between the atriopore 

 and anus, ao, aorta ; df, dorsal fin ; scl, sclerotome ; 7ff, ventral fin ; 

 the other lettering as above. (All the figures after Hatschek.) 



lateral grooves. The right outgrowth soon becomes con- 

 stricted off from the archenteron ; the left retains its connec- 

 tion with it for a long time. These cavities have a somewhat 

 complicated history. Their walls give rise to the first myotome, 



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