Morphogenesis of the ultimobranchial body and its colonizing cells in the chick embryo.

Abstract

The development of the ultimobranchial body and its colonizing cells was studied in chick embryos in Hamburger-Hamilton's stages 21 to 46. The most significant observations included the following: 1.-The ultimobranchial body is a separate morphological entity with particular characteristics distinguishing it from the fourth pharyngeal pouch in all developmental stages. 2.-The fifth pharyngeobranchial ducts disappear in Hamburger-Hamilton's stage 27. 3.-From Hamburger-Hamilton's stage 27 on, the left ultimobranchial body is in contact with the caudal end of the left parathyroid IV primordium. 4.-The group made up of the ultimobranchial body and its colonizing cells at no time fuses with the thyroid gland.