nner and outer 

 mesodermal 

 ngs 



Fig. 28. Gastrulation and later embryonic stages In Amphioxus (after Villee). 



cells, it is the primitive digestive tract (gut). The embryo now lengthens 

 and the site of the old animal pole becomes the anterior end of the em- 

 bryo; the site of the old vegetal pole becomes the posterior. 



The gastrula is still only two-layered and the third layer ( mesoderm ) 

 appears at this time. Three long tubes of cells are pinched off from the 

 endoderm. The middle one becomes the notochord, a flexible skeletal rod 

 which persists in amphioxus as its axial supporting member. (In higher 

 chordates, the notochord appears during embryogenesis but is later sur- 

 rounded or replaced by the vertebral column. ) The lateral tubes grow by 

 cell division into large tissue masses that are divided into segments along 

 the length of the embryo and are called somites. These expand and plaster 



58 



