STAEFISH 119 



yolk. This is the first external indication of 

 the division of the egg. What relation has the 

 position of this notch to that of the " direction- 

 cells " ? Can you suggest a reason why these 

 are called " direction-cells " ? Watch the notch 

 as it deepens into a furrow. In what direction 

 and how far does the furrow extend? How 

 long before it divides the yolk into two distinct 

 parts, blastomeres ? What is the shape of 

 each of these ? Can you distinguish a nucleus 

 in each part? Try reagents if necessar}'. 

 Does this division have any effect upon the 

 membrane ? Do all of the eggs with which 

 you have mixed sperm begin to divide at the 

 same time, and does the process go on with the 

 same rapidity in all ? 

 Draw. 



If reagents were applied while watching the first seg- 

 mentation, fresh specimens must be taken for the fol- 

 lowing stage : 



2. The second division. — How long before the 



second division begins ? What is the first in- 

 dication that a second furrow is beginning to 

 form? What is its direction with respect to 

 the first ? How long before the egg is com- 

 pletely divided by this furrow? How many 

 cells are there now inside the q^^ membrane ? 

 Draw different stages of the second segmentation. 



3. The third division. — Compare this with the first 



two as regards the various features mentioned. 

 Draw. 



4. The blastula. — After a few hours, during which 



successive divisions take place, the &gg will be 

 seen to consist of a solid spherical mass of cells 



