During the first four days after conception, the zygote undergoes rapid cell divisions and becomes multicellular. what is this cell division called?

The answer is cleavage. In embryology, cleavage is the division of cells in the early incipient organism. The zygotes of numerous species experience fast cell cycles with no noteworthy development, delivering a group of cells an indistinguishable size from the first zygote.

Ithink the answer is nuclear membrane

This movement is also referred to as  glacial plucking