At approximately day 1

Large clusters of lymphocytes identified.

Reference

Hahn von Dorsche H, Falkmer S. 2000. Ontogenesis of human islets of Langerhans. Review of light and electron microscopic data, immunohistochemical and functional data on fetal development of the endocrine pancreas. Journal of Evolutionary Biochemistry and Physiology 36(6):701-718.