The objective of this activity is to study the process of fertilization and early development in sea urchins. The sea urchin has continued as important material for both cell and developmental biologists, and together with the frog and mouse is the best material for investigation of many problems of development of deuterostomes, including cell division and cell-cell interactions.
The ripe sea urchin egg has completed meiosis, its polar bodies have disappeared, and it is shed in a completely mature state. The moment of fertilization is relatively easy to observe by the appearance of a halo, or fertilization envelope about the egg. At fertilization the egg undergoes a dramatic cortical reaction, marked by exocytosis of cortical granules and consequent formation of the fertilization envelope.
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In Strongylocentrotus purpuratus (purple sea urchin) pronuclear fusion will take place at about 25 minutes post fertilization at 15 oC. The first cleavage will occur at about 100 minutes. The succeeding divisions will then occur every 60 minutes.