CLEAVAGE I I 



Draw an eight-celled stage, lateral mew. Label the parts. 



Examine further stages of division. What is the 

 relative size of the cells in the dark and light hemi- 

 spheres? What is the relative number? 



Examine an egg in a many-celled stage. With a sharp 

 scalpel or razor split the egg into halves through the 

 poles. What is in the center of the egg? How many 

 layers of cells are there? When an egg has divided until 

 there are many cells and these cells are arranged in a 

 single layer so as to form a hollow sphere, it is called the 

 blastula stage. The cavity in the center of the sphere 

 is the segmentation cavity. 



Draw a section through a blastula and label the parts. 



Examine an egg which has a small indentation on one 

 side. Where is this indentation with reference to the 

 poles? With a sharp scalpel or razor split the egg into 

 halves through this indentation. How are the hemi- 

 spheres of this egg different from the hemispheres of the 

 blastula stage? What is the form of the segmentation 

 cavity here? How many layers of cells are there? The 

 cells at one pole divide more rapidly than they do at the 

 other pole, which results in the folding in of the cells 

 at one pole. Which pole? This process is known as 

 gastrulation, and the stage is known as the gastrula 

 stage. The indentation, or the cavity thus formed, is 

 called the archenteron or primitive gut cavity. The 

 opening of the archenteron to the outside is called the 

 blastopore. 



Draw a section of a gastrula and label the parts. 



