DEVELOPMENT 
91 
Eflg Nucleus Sperm cell' Sperm Nucleus Fused Nucleus 
Figure 76. — Diagram Illustrating Fertilization in Frog Egg. 
a, The sperm cell is penetrating the cytoplasm of the egg; b, the head of 
the sperm cell has become transformed into a nucleus; c, the egg-nucleus and 
the nucleus derived from the sperm head fusing. This fusing is fertilization. 
mass. These sperm cells make their way through the soft 
jelly and one of them must enter each egg or it cannot grow 
into a tadpole. 
As soon as the sperm cell enters the egg (Figure 76), it 
begins to change from a solid, pointed body into a round 
nucleus which is so much like the nucleus already in the 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 one nucleus in 
the egg (Figure 76). This 
last change is fertilization, 
which is defined as the 
union of the contents of 
the egg and the sperm 
nucleus. After this union 
is completed the egg be¬ 
gins to divide into cells, as 
shown in Figure 77, and 
finally a tadpole is grown. 
d e f 
Figure 77. — Dividing Egg of Frog. 
After the egg has been fertilized as 
shown in Figure 76, the embryo begins to 
divide in a regular manner, a, Two-celled 
embryo; b, four-celled embryo ; c, eight- 
celled embryo; d, thirty-two to forty-eight 
celled embryo; e, many celled stage; 
f, embryo beginning to form central nerv¬ 
ous system. 
