Besides the placenta and yolkless eggs, early mammalian development has many more unique features:
Rotational cleavage. Unlike, for example, sea urchin eggs which undergo rotational cleavage, with the first two cleavage planes being meridional and the third one being equatorial, the second mammalian cleavage divides one blastomere equatorially and the other one meridionally. This pattern continues, alternating between equatorial and meridional cleavages throughout the early cell divisions.
Because of this, there is early loss of synchronicity of cleavage, meaning that unlike most embryos, the early embryo of mammals can have an odd number of blastomeres during cleavage.
Due to the small volume of their eggs, mammal embryos cannot supply the dividing blastomeres with maternally pre-formed mRNA, ribosomes, and proteins normally stored in the egg cytoplasm. Instead, mammals activate transcription and translation very early in their blastomeres, resulting in slowed down early development.
This also means that, because they are producing their own molecules so early in development, mammal embryos start growing earlier than most of other early embryos, making up for small initial size.
Finally, although it is not surrounded by a hard shell, the mammalian egg still has a protective envelope from which the embryo has to hatch in order to attach to the uterus and develop the placenta. To do so, the mammal embryo undergoes compaction of blastomeres at the 8-cell stage. Compact blastomeres are better protected when they hatch inside the maternal body.
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- current answer: <p id="">Mammals can produce <blank1> number of blastomeres during <blank2>. This is due to <blank3> cleavage of <blank4> eggs.</p>
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number of blastomeres during
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. This is due to
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cleavage of
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