SECTION X. EMBRYOLOGY 



Within the limits of this manual, it is possible to give direc- 

 tions for a rapid survey of the principal external features 

 only of the development of the frog. This survey can be 

 performed most advantageously in the spring when living 

 material is available. But as this is not always feasible, the 

 directions are based on preserved material. This will be 

 given you in watch-glasses to be studied with the aid of a 

 hand-lens. Be careful in handling the material. When you 

 desire to change the position of an egg or embryo do so by 

 using a gentle stream of water from a pipette. 



1. Cleavage and Germ Layer Formation 



Examine the jollowing stages. 



329. The fertilized egg before cleavage (one-cell stage). 

 Note particularly: the egg-envelope (jelly), the 

 apical (animal) hemisphere with its black pig- 

 ment, the white abapical (vegetative) hemisphere. 

 It is sometimes possible with fresh material to see 

 the small polar bodies at the apical pole, and the 

 gray crescent between the apical and abapical hem- 

 ispheres. (Demonstration of spermatozoa may be 

 shown at this time.) 



330. The first cleavage (two-cell stage). Note the first 

 cleavage furrow. What relationship has it to the 

 apical pole (and the gray crescent, if present)? 



33L The second cleavage (four-cell stage). Note the 

 first two cleavage furrows. What is their relation- 



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