Like other urchins, the red sea urchin has a dome-shaped shell called the “test” covered in spines. Its test is also covered with tube feet that it uses to move and bring food to its mouth, at the ...
By about 1900 the observations of Hertwig, Fol, and others were brought together to make the now-familiar picture: a single sperm cell penetrates the egg cell; the nucleus of the sperm merges with the ...