222 



PRINCIPLES OF ANIMAL BIOLOflY 



The mtithod of formation of the nicsoderin differs in different animals. 

 In Amphioxus the upper lateral portions of the endoderm (Fig. 177) are 



ar 



Fig. 176. — Gastrulation in the telolecithal embryo of the lamprey. A, B, C, D, E, 

 successive stages; ar, archenteron; b, blastoccElo; bp, blastopore; d, line of delamination; 

 dig, digestive tract; cc, ectoderm; en, endoderm; Li, liver. (After Glaesner, courtesy of 

 Gustav Fisher.) 



turned outward in the form of grooves, shown in cross-section in the figure. 

 The edges of each groove meet and fuse, and the groove now in the form 



Fig. 177. — Mesoderm formation in Amphioxus. A, evagination of the upper lateral 

 portions of the endoderm; B, separation of the mesodermal pouches from the endoderm; 

 c, ccelom; dg, digestive tract; ec, ectoderm; en, endoderm; m, mesoderm; mp, mesodermal 

 pouch; n, neural tube; nd, notochord. (After Hatschek.) 



of a tube is completely separated from the endoderm. The two tubes 

 thas formed are the mesoderm, and the slender openings in them consti- 

 tute the body cavity, or ccelom. In later stages of development the tubes 



