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In contrast , historical literature on most other arthropods reports that PGCs arise from mesoderm during midembryogenesis , suggesting that an arthropod last common ancestor may have specified PGCs via zygotic mechanisms .
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Please list all entity words in the text that fit the category.Output format is "type1: word1; type2: word2". 
Option: Chemical 
Text: {0} 
Answer:
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