Figure 2.

Electron microscopy of ER and Golgi in sea urchin embryos (5 h after fertilization). Top panel, ER is accumulated in the region of the mitotic poles. Yolk platelets (large, dark-staining organelles) are excluded, but some mitochondria are present (magnification, 2070×). Middle panel, higher-magnification view of the accumulated ER in the region of a mitotic pole (magnification, 21,360×). Bottom panel, typical appearance of a Golgi stack. Golgi stacks are scattered throughout the cytoplasm and do not appear to be connected. The Golgi stack shown here is ∼1.2 μm long in its widest dimension. The nucleus is seen on the left, indicating that this blastomere was fixed during interphase (magnification, 18,000×).