Modes of Gastrulation in Vertebrates

The modes of gastrulation in the different classes of Vertebrates are discussed from a comparative point of view, with the particular aim of finding some system which avoids postulating a sharp dualism between the anamniote telolecithal forms, in which the edge of the blastoderm consists of presumptive endoderm, and those of the Am-niotes, in which it is ectodermal. It is suggested that the most profitable way of regarding all blastodermic forms is to envisage them as derived from the basic type of blastula by a process in which the roof and floor are squeezed together, so that a flattish double-layered plate is produced. The concept of the 'primitive edge ' of such a blastoderm is introduced, and it is suggested that in the amniotes the presumptive ectoderm has expanded greatly beyond the primitive edge, while there has also been a strong convergence towards the dorsal side. The conditions in certain highly yolky amphibian eggs are shortly described, and it is argued that they show conditions similar to those which would be expected on the hypothesis of a squeezing together of the blastula roof and floor. IT is during gastrulation that the fundamental plan of the vertebrate bodyis brought into existence, and one of the most important tasks of compara