From cleavage to primitive streak formation: A complementary normal table and a new look at the first stages of the development of the chick

A manual method has been developed to extract eggs from the genital tract without harming the hen. Fourteen developmental stages preceding Hamburger and Hamilton's stage 2 have been studied from live material and photographed from both upper and lower surfaces. Three developmental periods have been distinguished: cleavage, formation of area pellucida, and hypoblast formation. The last two are morphogenetic periods which follow the determination of the embryonic axis. During cleavage the diameter of the germ becomes reduced and the cytoplasmic mass becomes thicker. Later there is a progressive increase of the germ's area. The formation of the area pellucida is a result of an oriented massive loss of the yolk laden cells of the lower layers. The formation of the primary hypoblast is a result of posterio-anteriorly oriented polyinvagination and a later coalescence of the cell aggregates in the same direction. It is not yet possible to conclude whether there is also an anteriorly directed cell movement. The inconsistent usage of the terms blastodisc and blastoderm is discussed. The authors suggest avoiding the term blastodisc and using the general term germ for all the early stages. The term blastoderm may be applied from stage VI onward.