Erythrocytes might the best of example of form equals function because they lack many organelles. What organelles are absent in erythrocytes?

Mature mammalian red blood cells lack nuclei, mitochondria and rough endoplasmic reticulum, they do not contain DNA and consequently, cannot divide. They also cannot synthesise RNA nor synthesise any new proteins, and consequently, have a limited lifespan. Mature red blood cells circulate for about 100–120 days in the body before they are removed by the spleen.

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The answer is observations and interpreting.