State of Maturation of the Egg at Fertilization

Fertilization may take place at different stages of the maturation of the egg. The two extreme types are exemplified by the sea-urchin and Ascaris.

In the sea-urchin, fertilization takes place of the fully mature egg, with both polar bodies formed. Immediately after the entry of the sperm the two pronuclei conjugate and fuse to form the fusion nucleus. Because of the short interval between sperm entry and nuclear union the two pronuclei are unequal in size at the time of union. The sperm aster divides into two only after nuclear fusion. Complete fusion of the male and female pronuclei takes place and maternal and peternal elements cannot be distinguished.

In Ascaris the egg is immature at the time of sperm entry, with no polar bodies formed. The sperms nucleus has to wait in the egg until both divisions have been completed. Therefore it has time to mature, and at the time of nuclear union is equal in size to the egg pronucleus. The sperm aster divides into two before nuclear fusion. Because each pronucleus has already formed chromosomes, maternal and paternal elements from distinct groups. There is no true fusion nucleus.



The sea-urchin and Ascaris type of fertilization represent extremes which are connected by intermediate stages. In Chaetoppterus sperm entry takes place during metaphase of the first maturation division. In the annelid Ophryotrocha it takes place during anaphase. In Amphioxus and most chrodates the sperm enters the secondary oocyte, after the formation of the first polar body.

The path of the pronuclei in the egg

Only the head and the middle piece of the sperm enter the egg: and tail is left outside. The sperm enters with the head containing the nucleus in front and the middle piece behind. In many forms the sperm undergoes a rotation through 180, so that the middle piece come to lie in front of the head. The path taken by the sperm through the egg has been studied in the frog. The sperm enters near the animal pole and its initial path is called the penetration path. It then changes direction to meet the egg nucleus (copulation path). The zygote nucleus then moves to the definitive position of the first cleavage nucleus. This path is called the cleavage path.

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