Study of the development of the internal organs of the double malformations of Chironomus dorsalis by fixed and sectioned materials

Abstract

ABSTRACT The development of the internal structures was investigated by fixed sections of the ‘double cephalon’ and ‘double abdomen’ of Chironomus dorsalis. The cell proliferation that gives rise to ‘germ Anlage’ or embryonic rudiment begins, in the double cephalon, along the entire convex (ventral) side of the egg and, in the double abdomen, at both ends of the flat (dorsal) side. As a result, a single fused Anlage of the double cephalon appears along the entire convex side of the egg and two germ Anlagen of the double abdomen appear at both ends of the flat side. During the formation of the germ band, both the posteriormost part of the double cephalon which lies at the middle of the convex side of the egg and the anteriormost part of the double abdomen which is located at the middle of the convex side, fail to differentiate and later degenerate. In each of the duplicated heads of double cephalon, cephalic segments anterior to the first maxillary segment are formed, but the thoracic and abdominal segments are entirely missing. In each half of the double abdomen, eight abdominal segments posterior to the second abdominal segment are produced and the cephalic and thoracic segments are omitted altogether. The two pairs of mid-gut rudiment from both halves of the double cephalon are temporarily united but they break apart by the end of the blastokinesis. When the two pairs of mid-gut rudiment from both halves of the double abdomen meet, they remain fused with each other, being surrounded by the visceral mesodermal cells in the normal way, and develop into the mid-gut epithelium. In the double malformations, the pole cells are contained in only one member of the duplicated structures. The pole cells of the double cephalon develop into the tetra-nucleate state (Hasper’s second step), but they fail to fuse to form the gonad. In the double abdomen, the gonad develops in the one abdomen containing the pole cells and no replacement occurs in the sister abdomen without the pole cells. The embryonic envelopes of the double cephalon do no’ retract into the interior of the embryo, while they do in the normal way in the double abdomen. The double cephalon can never hatch but the double abdomen can emerge.