Studies on Embryonic Determination of the Harlequin-fly, Chironomus dorsalis : I. Effects of centrifugation and of its Combination with Constriction and Puncturing

Abstract

By centrifuging, alone or in combination with constricting and puncturing of the egg, the embryonic determination in the harlequin-fly C. dorsalis has been studied. In eggs centrifuged moderately, three zones were distinguished: a centrifugal yolk zone, a centripetal oil zone, and an intermediate zone consisting of cytoplasm with nuclei. The mode of redistribution of the constituents of the centrifuged eggs differs according to the developmental stage at the time of centrifugation and to the centrifugal force. The strata of weakly centrifuged eggs begin to mix with each other immediately after centrifugation. Redistribution of the constituents of strongly centrifuged eggs always occurs in a definite way, even though the redistribution after centrifuging at the nuclear division stage does not take place until later, in the nuclear migration stage, while the cytoplasm of eggs centrifuged during and after nuclear migration begins to migrate as soon as the centrifuge is stopped. By centrifuging the eggs at the nuclear division stage, various types of malformation related to the direction of centrifugation are caused. These malformations are characterized by having duplicated parts. The frequency of occurrence of the malformations increases with increase in the centrifugal force. The structure of the double monsters is further modified by separating a part of the yolk or oil in the centrifuged eggs by strong centrifuging or by constricting, and by removing a part of the cytoplasm from the centrifuged eggs by puncturing. Modification always occurs in the mid-part of the double monster in the form of loss of segments. The number of omitted segments depends on the volume of the separated or the removed part. By applying two successive centrifugations in reverse directions it was found that a causal relationship between the direction of centrifugation before nuclear migration and the resulting malformation type is established at about the nuclear migration stage. From late centrifugation, two other groups of malformations resulted which are different from those obtained by early treatment. Those of the first group, which were caused by centrifuging at the syncytial blastoderm stage, are characterized by having a defect on the centrifugal side and no duplicated parts. Those of the second group were obtained from eggs centrifuged at the cellular blastoderm stage. In the extreme degree of this malformation the cells which reach the final level of differentiation are not organized into an embryo, forming instead small tissue masses. The specific features of the embryonic development of C. dorsalis, particularly thorax development, and the more general problem of the relation of the mode of development of Chironomus to that of other Dipterans, are discussed.