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 equalit}^ of the first two cells ma}^ be better appre- 

 ciated when we know that if one of these be destroj^ed by 

 a touch from a red-hot needle, a perfect half 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 j-olk, 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 

 5^olk or egg has but just left the ovary a tiny dot is visible 

 on one side, — the germinal vesicle, w^hich after fertilization 



