Fig. A Sagittal section through the head of Ciona intestinalis spermatozoon. Although the acrosome reaction is almost completed, the mitochondrion is still applied to the nucleus. Bar: 1um.
Fig. B Enlargement of the anterior tip of the head in A. A vesicle of fused membrane is probably being shed. Bar: 200nm(this is also applicable to C-G).
Fig. C Horizontal section through the anterior region of the head.A vesicle is seen at the anterior most region.
Fig. D Sagittal section through the head. The acrosome reaction is completed. Bar: 1um.
Fig. E Enlargement of the anterior region of the head following the acrosomereaction. The acrosome has disappeared fron its proper location.
Figs. F,G Serial horizontal sections through the head apex, following the completion of the acrosome reaction. AS, apical substance.

for details: J. Structural Biology 111, 77-83 (1993)