58 EMBEYOLOGY OF THE LOWEE VEETEBEATES oh. 



itself becomes nipped off from the remainder of the archenteron 

 (Fig. 34, C). 



The original archenteron is now replaced by a main portion, the 



Fig. 35. — Transverse sections through embryos of Lepidosiren to illustrate the origin of 



the mesorlerm. 



A, stage 12 ; B and C, stage 14. ect, ectoderm ; end, endoderra ; ent, enteric cavity ; vies, mesoderm ; 



N, notochordal rudiment. 



enteron, the wall of which — the definitive endoderm — will eventu- 

 ally become the lining epithelium of the alimentary canal, and, lying 

 dorsal to this on each side, a series of closed sacs, or practically solid 



