Determination of animal and vegetal pole How do we determine which is an animal pole and which is a vegetal pole in a zygote? My professor said that the zygote can be divided into two hemispheres, vegetal and animal poles. The hemisphere with yolk is the vegetal pole and the one without is the animal pole. But I couldn't find an authentic book that I have access to saying anything about the polarity of the zygote. However I stumbled upon this paragraph from Objective Zoology and I'm not sure if it's right. > Zygote or egg of frog is spherical and measures about 1.6mm in diameter. The upper half of the zygote is black (pigmented) and contains nucleus and cytoplasm. The lower part of the zygote is white or light yellow in colour and contains the reserve food in the form of yolk. The upper pigmented part is called the animal pole and the lower part vegetal pole.

To resolve this question here's something I found in Developmental Biology by Gilbert $-$

> The amount and distribution of yolk determines where cleavage can occur and the relative size of the blastomeres. When one pole of the **egg** is relatively yolk-free, the cellular divisions occur there at a faster rate than at the opposite pole. _The yolk-rich pole is referred to as the **vegetal pole** ; the yolk concentration in the **animal pole** is relatively low._