ABSTRACT Our present knowledge of the problematical ‘dorsal organ’ of Collembolan embryos is based mainly on the writings of Uljanin (1875), Lemoine (1883), Wheeler (1893), Claypole (1898), Uzel (1898), and Philiptschenko (1912). Owing to the minute size of the embryos of most Collembola much difficulty has been experienced in making accurate observations on its structure ; only in the work of Claypole and of Philiptschenko have actual sections been employed for the purpose.