The effect of transection of the duck blastoderm on the orientation of the embryo.

Von Baer (1828) formulated a rule according to which the cephalo-caudal axis is perpendicular to the long axis of the shell, and the head of the embryo is directed forwards when the egg is placed with the blunt end to the left. This law has been checked in several species of bird: hen, duck, pigeon, goose, turkey (see Clavert, 1960, for references) and quail (Fargeix, 1963, 1964). Embryos obeying von Baer's rule were found to constitute the most numerous class, but considerable interspecific as well as intraspecific differences were observed. The studies by Lutz (1949) supply extensive information on the regulative capacities of the duck blastoderm, but do not provide any data concerning the determination of the presumptive axis of the blastoderm. The control material of this author revealed a large percentage of embryos obeying von Baer's rule. In consequence Lutz defines transections perpendicular to the long axis of the shell as 'parallel to the presumptive axis', and transections parallel to the long axis as 'perpendicular to the presumptive axis'. Vakaet (1962), however, has already drawn attention to the fact that transection of the duck blastoderm does to a certain extent affect the orientation of twin embryos. The experiments presented below further investigate this observation, the direction of transection affecting the orientation of the embryonic axis even in cases in which only one embryo developed.