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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.

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    $\begingroup$ "Vegetal" and "animal" poles are archaic terms. As pointed out by Elaine, the polarity axis is defined by ectoderm and mesoderm. There are other kinds of polarity as well. You can read more about it, for different organisms, from Developmental Biology by Gilbert. $\endgroup$ – WYSIWYG Nov 8 '16 at 7:04
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Do you have any reason to doubt this statement? The wikipedia article says pretty much the same thing. When reading the folllowing, don't forget that one can talk about such poles at different stages of development.

The animal pole consists of small cells that divide rapidly, in contrast with the vegetal pole below it. In some cases, the animal pole is thought to differentiate into the later embryo itself, forming the three primary germ layers and participating in gastrulation. [..] The vegetal pole contains large yolky cells that divide very slowly, in contrast with the animal pole above it. [..] The animal hemisphere is dark brown, and the vegetal hemisphere is only weakly pigmented.

and it comes with the following picture

enter image description here

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  • $\begingroup$ You know, wiki article talks about polarity of an embryo and not a zygote. I haven't encountered any authentic book saying that they are the same, so is my question. $\endgroup$ – Tyto alba Nov 6 '16 at 17:07
  • $\begingroup$ Molecular Biology of the Cell, 6th ed, p. 1156 i.imgur.com/ZNTT53Oh.jpg After sperm entry and rotation of symmetrization, the zygote has an animal and vegetal pole. It also says elsewhere that the name animal and vegetal are due to old beliefs, and roughly correspond to what will be inside (endoderm and mesoderm) and outside (ectoderm) respectively, after gastrulation. The v-a polarity is established by gradient of stocked mRNA and vitellin. Mecanisms vary extensively between phylum. $\endgroup$ – Eliane B. Nov 6 '16 at 17:28
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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.

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