The third, or 'middle' layer of the developing tissues of the early embryo, forming between the ectoderm and the endoderm during the transformation of the embryo known as gastrulation; mesoderm gives rise to the body's muscles, bones and various connecting tissues.

A step of major reorganisation of the developing embryo, coinciding time-wise with the missing of the menstrual period in the early pregnant woman, in which the third, or 'middle' layer (the mesoderm) forms. Occurs after blastulation (the transformation giving rise to the blastocyst) and before neurulation (the transformation forming the brain and spinal cord).