The study of serial sections allows for the study of the microscopic anatomy of the embryo, in particular, the internal anatomy of the embryo. Many features that are not evident in the study of the whole embryo can be examined by viewing a sequence of sections of the embryo.
Serial sections consist of the entire embryo sectioned in a certain plane.
In the case of serial cross sections, the entire embryo was sectioned perpendicularly to the cranio-caudal axis from the beginning of a portion of the head to the end of the tail and all the sections were laid out in sequence on microscope slides. The embryo from which the accompanying photomicrographs were taken consisted of 380 sections. It is important to note that by 72 hours of incubation, the shape of the chick embryo has become more complex than its earlier straight line configuration. From 33 hours to 72 hours the embryo has undergone rapid growth of its anterior end and has developed both a cranial flexure and a dextral torsion of its anterior end. A flexure is a ventral bend in the cranio-caudal axis of the embryo and the cranial flexure is a bend at the midbrain region.