128 GENERAL CONCEPTS 



Ectode-rxn-i 



Entoderm 



Ectoderm 



Archente-ron 



Archenteron- 



Caudal 



Entoderm/' ""Bla-stopore 

 •Weural plate- rCoe.lomic pouch. 



^Ectode-rni' 

 ^Entoderm'' 



Keural oroo\^e 



Nofco chord 



Gut 



Archenteron^ 



Neural tube 



Somite 



Somatopleure 

 Myotome 

 iplanclinopleure 



Figure 6.8. Stages in gastrulation and mesoderm formation in Amphioxus. Note 

 that the mesoderm forms by the budding of pouches from the archenteron. 



duced gastrulation. The outer of the two walls of the gastrula is the 

 ectoderm, which eventually forms the skin and nervous system. The 

 inner layer, lining the archenteron, is the endoderm, which will form the 

 digestive tract, liver, pancreas and lungs. 



Cleavage and gastrulation are markedly modified in telolecithal 

 eggs by the presence of the large amount of yolk. In the frog egg, which 

 may be called moderately telolecithal, the cleavage divisions in the 

 lower part of the egg are slowed by the presence of the inert yolk. The 

 resulting blastula consists of many small cells at the animal pole and a 

 few large cells at the vegetal pole. The lower wall of the blastula is 

 much thicker than the upper one and the blastocoele is flattened and 

 displaced upward. Only the small disc of cytoplasm at the animal pole 

 of the hen's egg undergoes cleavage divisions; the lower, yolk-filled part 

 of the egg never cleaves. As a result, the blastocoele is simply a shallow 

 cavity under the dividing cells. Gastrulation occurs in both frog and 

 chick egg, and an archenteron is formed, but the process is greatly 

 modified by the presence of the yolk. Gastrulation in the frog involves an 



