At approximately day 1

Thyroid gland approaches structural maturity.

Reference [PubMed Link]

Fisher DA. 1997. Fetal thyroid function: Diagnosis and management of fetal thyroid disorders. Clinical Obstetrics & Gynecology 40(1):16-31.

At approximately day 1

Islets develop with insulin cells in the center and glucagon, endorphin and pancreatic polypeptide cells surrounding them by week 20.

Duration: 3 weeks

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.