ONE OF THE earliest embryonic stages after the fertilized egg implants into the uterine wall is the gastrula, when the fertilized egg has gone through several divisions and the cells can be classified into three groups that have first begun to show differentiated characteristics. These groups are called germ layers and represent some of the first lineage specific stem cells in embryonic development. These differentiated cells can be grouped into three types, which are layered across the gastrula. Each layer, called a germ layer, will eventually become certain types of tissues in the adult. These layers are the endoderm and ectoderm, and in between them the mesoderm.

[Page 228]The ectoderm, the outermost of the three germ cell layers, eventually develops into the epidermis and hair

The endoderm ...