The jelly canal marker of polarity for sea urchin oocytes, eggs, and embryos

The jelly coat of a sea urchin egg possesses a narrow conical channel which identifies the animal pole. This rarely seen structure was first reported by Boveri in 1901 and is easily demonstrated by immersing freshly ovulated eggs into an ink suspension. The jelly canal, as this feature is called, fills with ink particles as the jelly swells. The jelly canal occurs on full-sized primary oocytes and unfertilized eggs. When filled with ink it serves as a useful marker of the animal pole during fertilization and early development when it is not otherwise visible. A common synonym for the jelly canal is ‘micropyle’, but this is a misnomer because sperm do not selectively pass through it for fertilization. The presence of the jelly canal on oocytes suggests how it might form and does not prove that the animal-vegetal polar axis in sea urchin eggs is defined before meiotic maturation.