DEVELOPMENT 



121 



egg cell that none but experts in this study can tell which 

 came from the sperm cell and which from the egg cell. 

 These two nuclei come in contact and unite, leaving but 



Figure 118. — Frog Eggs. 



one nucleus in the egg (Figure 119). This last change is 

 fertilization, which is defined as the union of the contents 

 of the egg and the sperm nuclei. After this union is 

 completed the egg begins to divide into cells, as shown 

 in Figure 120, and finally a tadpole is grown. 



Eg'g Nucleus Sperm cell Egg' Nucleus Sperm Nucleus Fused Nucleus 



Figure 119. — Diagram Illustrating Fertilization in Frog Egg. 



As soon as the young tadpole hatches, it attaches itself 

 to plants and lives for the first few days upon the food- 

 yolk within its own body ; the mouth forms, and horny 

 jaws develop. Then the tadpole begins to feed upon 



