4.6 The fertilization is complete.
The formation of the zygote

The formation of the zygote

After the two pronuclei have come as close together as they can, no merging of them takes place, i.e., a fitting together of the chromosomes of the two pronuclei within a single nucleic membrane does not happen. It is much more accurate to say that the nucleic membranes of both pronuclei dissolve and the chromosomes of both align themselves on the spindle apparatus at the equator.
The zygote, the first cell of a new organism with an individual genome (2n4C) is created by the alignment of the maternal chromosomes together with the paternal ones on a common spindle apparatus.

Fig. 49 - A zygote  Legend


Nucleic membranes of the
pronuclei, as they are dissolving
Microtubules of the mitotic spindle

Fig. 49
22 hours after the fertilization a mitotic spindle has formed (more info).
The nucleic membranes of the two pronuclei dissolve. This is the visible sign that the zygote has been created.

The mitotic spindle divides the chromosomes that have just been brought together into the two first cells of the embryo. This proceeding towards the two-cell stage occurs on average between 22 and 26 hours after fertilization. Fig. 50 - The zygote divides  Legend

Fig. 50
23 hours after fertilization: the spindle apparatus in the anaphase; the chromosomes are pushed apart. (More info with videos 620 kB or 940 kB)

Commentary and summary:

The zygote - by definition the first cell of the embryo – undergoes only an incomplete cell cycle.
With the penetration of the sperm cell, the genetic information of the two parents are not instantly brought together in order that they are then duplicated together and distributed in the subsequent mitosis. In reality, the paternal and also the maternal genetic information are first duplicated in two independent pronuclei and also there condensed again into chromosomes so they can align themselves along the equator of the mitotic spindle.

The goal of the fertilization cascade is thus achieved:
  • The fabrication of a diploid set of chromosomes
  • The determination of the chromosomal gender of the new individual
  • The induction of normal "cleavage division" for embryogenesis.

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