On his website he writes: «Embryonic stem (ES) cells are pluripotent stem cells derived from inner cell mass of mammalian blastocysts. (faith.org.uk)
The two broad types of mammalian stem cells are: embryonic stem cells that are isolated from the inner cell mass of blastocysts, and adult stem cells that are found in adult tissues. (stemcelltreatments.in)
Gardner DK, Lane MW, Lane M. Development of the inner cell mass in mouse blastocysts is stimulated by reducing the embryo: incubation volume ratio. (adelaide.edu.au)