THE DEVELOPMENT OF THE INDIVIDUAL 



525 



To aid in visualization, compare the embryo with a hollow, soft 

 rubber ball into which you have pushed your finger to cause an invagina- 

 tion. This invaginated embryo is called the gastrula. The invagina- 

 tion continues until it reaches all the way across the ball. The embryo 

 is now shaped like a tube within a ball. The tube is called the 

 archenteron and is destined to form the inner portion of the alimentary 

 tract. The opening of the archenteron at the point of original invagina- 

 tion is called the blastopore and will eventually be the region of the 

 anus. The cavity within the archenteron is the gastrocoel. There are 

 now two distinct body layers — the outside layer of cells is known as the 

 ectoderm, and the cells forming the tube within the ball make up the 

 endoderm. This stage of development is suggestive of the diploblastic 

 type of animals that never have more than two body layers even in their 

 adult form. When a hydra contracts into a little ball it is somewhat 

 like an embryo of a chordate at this gastrula stage. 



Formation of the Mesoderm and the Coelom 



Since the chordates are triploblastic animals, there is still a third 

 layer, the mesoderm, which must be formed before the various body 



Neural plate 



Archenteron 



Som ites 

 (mesoderm ) 



Spinal cord 



Notochord 

 Coelom 



Gut 

 (enter on) 



Fig. 33.2. Formation of the mesoderm, coelom, and nerve cord in Amphioxus. These 

 drawings represent a cross-section through the gastrula and post gastrula as the 

 somites and notochord pinch oft from the endoderm while the spinal cord is formed 



from an invagination of the ectoderm. 



