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enter image description here Could you tell me what the yellow mass within the blastocoel is called ? I know that blastocoel is a fluid filled cavity, so are the yellow coloured cells the inner part of blastoderm or are they some other cells ?

If they are the part of blastoderm itself, why is it not in the same colour of light-orange as the blastoderm, or is it the mesoderm, or the inner cell mass ?

And is it the blastoderm that forms ectoderm , and endoderm pinches off the ectoderm during gastrula phase? And could you also explain how the orange mesoderm formed ?

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The blastocoel, as you say yourself, is a fluid-filled cavity (wikipedia) so it has no cells. Fig. 1 shows cellular and non-cellular regions of the blastocyst.

Your questions on the formation of the three germ layers during germination are nicely shown in the illustration in Fig. 2, right two panels.

blastocyst
Fig. 1. Blastocyst. source: wikipedia

germination
Fig. 2. Formation of the germ layers. source: Class Connection

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  • $\begingroup$ could you also answer whether the yellow cells within the blastula are cells of blastoderm itself or are they some other type of cells? And also whether purple cells shown within the archenteron are cells of endoderm itself ? $\endgroup$ – Kaberi Ghosh Apr 4 '17 at 12:08
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    $\begingroup$ Just wanted to add, OP's image of blastula lacks inner mass so (without any citation) it is likely referring to Amphioxus blastula. $\endgroup$ – Tyto alba Apr 5 '17 at 12:43

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