• (n.) The more transparent cells, which finally become external, in many segmenting ova, as those of mammals.
A blastomere (one of the cells formed during the first few divisions of the zygote) which takes part in forming the ectoderm (the outermost tissue layer of an embryo). ... (09 Oct 1997) ... 
(ek´to-mēr) one of the blastomeres taking part in formation of the ectoderm.
Ec'to·mere noun [ Ecto- + - mere .] (Biol.) The more transparent cells, which finally become external, in many segmenting ova, as those of mammals. 
Type: Term Pronunciation: ek′tō-mēr Definitions: 1. One of the blastomeres involved in formation of ectoderm.