The most common and most likely answer to this crossword clue is the 8 letter word GASTRULA
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The straight part
We know that "embryo" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case embryo = gastrula.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does GASTRULA mean?
GASTRULA noun- A stage in the development of embryos of most animals consisting of a three-layered sac of ectoderm, mesoderm and endoderm.