Answer
Fig. 18.1
A nucleus.
Work Step by Step
Platelets are not cells but fragments large bone marrow cells called megakaryocytes. Erythrocytes start out as nucleated cells (erythroblasts) but lose their nucleus as they mature. They become biconcave discs with a depression where their nucleus was located. This shape enables erythrocytes (RBCs ) to fold and squeeze through narrow vascular channels .
