Light micrograph of a mouse embryo, taken at the end of the first mitotic division of the unicellular zygote, the recently fertilised ovum. From now on the 2 almost fully-formed cells will divide into 4, then 4 into 8, and so on, to form a ball of cells known as the morula. As the number of cells, called blastomeres, increases the individual blastomeres become smaller, so that the overall size of the cellular aggregate does not increase at first. The well-defined exterior wall around the embryo is the zona pellucida, a protective glycoprotein layer. Magnification: X 320 at 10x8 size.