Question

Which feature is shown ONLY by higher invertebrates, as compared to lower invertebrates?

Answer 1

Early on in development, the mass of undifferentiated cells that make up the embryo must take the first steps in deciding how to arrange themselves into component parts to eventually go on to form a fully developed body. This is a process known as 'gastrulation.' During this stage, the cells group into three layers, the first is the 'ectoderm' which then in turn generates the 'mesoderm' and 'endoderm' layers. In higher vertebrates, such as mammals and birds, the mesoderm and endoderm are generated from an axis running through the centre of the embryo. However, in lower vertebrates, such as amphibians and fish, the two layers are generated around the edge of the embryo.