466 The Bird 
the tadpole wriggles his way through the cloudy mass of 
gelatine and swims into the water. 
The first steps of this dividing or cleaving of the original 
single cell is similar in all eggs. The deep significance of 
the equality of the first two cells may be better appre- 
ciated when we know that if one of these be destroyed by 
a touch from a red-hot needle, a perfect halj tadpole will 
develop from the other unharmed twin cell. If we observe 
the cleavage of the whiter portion of the frog’s egg, we will 
notice that the furrows, though ultimately extending all 
the way around, yet grow very slowly in that portion. This 
is because much of the white part consists of yolk, or true 
food-matter, the more active formative material being 
confined to the black portion. 
If we follow this segmentation of the cells for some 
time, the egg of the frog will come to look like a diminutive 
blackberry—a single layer of cells thickly covering its 
entire surface, like the rounded protuberances of the berry. 
Now a curious thing happens. A tiny nick appears in one 
side, which gradually deepens and widens until it extends 
deep into the egg, pressing two rows of cells into close 
proximity to each other. This will be perfectly clear if 
we take a small rubber ball and squeeze it until one hollow 
hemisphere is pressed into the other. This stage of em- 
bryological life is called the gastrula, and is of the greatest 
significance, as we shall soon see. 
Without further comment at present, let us now leave 
the frog’s egg and consider that of the fowl. When the 
yolk or egg has but just left the ovary a tiny dot is visible 
on one side,—the germinal vesicle, which after fertilization 
