EN
The embryonal growth of Ornithonyssus bacoti (Hirst, 1913) is compared with that of Pergamasus brevicornis Berl. The common features were found to be: type of germ band, organization of the mouth apparatus, regressive revolutions and the development of the IV pair of perineal crura, whereas the distinguishing ones are: lower number of regressive revolutions of the embryo and the modus of dropping the egg membranes by larvae.