6), continue to receive Delta for two more cleavages and at eighth cleavage the upper Veg2 cells are removed from Delta as a consequence of a meridional cleavage, and because these cells receive fewer than 3В h of continuous Delta signaling, these cells become endoderm, while the lower tier of Veg2 cells continue to receive Delta for 3В h and as a consequence become the non-skeletogenic mesoderm ( Croce & McClay, 2010). The lower macromere progeny, the Veg2 cells (orange cells in Fig. 6, and the upper progeny of division of macromeres at sixth cleavage), are removed from the source of Delta signaling and proceed to become endoderm (though some Veg1 progeny also become ectoderm). At sixth cleavage the Veg1 cells (the yellow cells in Fig. 6) begin specification as endomesoderm through a combination of Wnt and Delta-Notch signaling. Initially, macromeres and their progeny (yellow and orange cells in the diagram at top left of Fig. 6 is a GRN model of endoderm specification. At a molecular level, much has been learned about how the endoderm cells are specified prior to and during invagination. Those endoderm cells move toward the blastopore, pass through it, and contribute to the lengthening archenteron. Leslie Slota, in Current Topics in Developmental Biology, 2020 5 Specification of endodermДЇollowing the initial inbending by the NSM, the archenteron lengthens by addition of endoderm cells from the vegetal plate. Slightly later the activating action also decreases markedly.Дavid R. Hoessels (1957) could show that its transforming inductive action is likewise gradually lost, leaving only its activating action to express itself in the induction of forebrain structures. This means that the presumptive prechordal endomesoderm gradually loses its notochord-forming capacity. The highest notochord-forming capacity is found in the dorsal blastoporal lip at the beginning of gastrulation (the presumptive prechordal endomesoderm) and shifts caudally toward the presumptive anterior notochordal portion of the archenteron roof during invagination. This phenomenon was further studied in detail by Takaya ( 1953a, b), Kato and Okada (1956), Hoessels (1957), Kato ( 1958, 1959, 1963a, b) and Masui (1960a). Masui (1960a) showed that preculture in Li solution prevented the change in differentiation tendencies and inductive power. If brought directly into contact with neural plate it again formed trunk axial structures. This was also achieved when the uninvaginated material was combined with noncompetent neurula ectoderm ( Kato and Okada, 1956). The above-mentioned change in differentiation tendencies and inductive capacity of the presumptive prechordal endomesoderm can also be affected when the material, before bringing it into contact with competent ectoderm, is first cultured in Holtfreter solution for 10 hours at 21вЊ, a period during which the material would have invaginated if left in situ. (The foregut structures comprise the pharynx, the gill pouches, and sometimes also stomach and liver.) In contrast, the presumptive notochordal region of the archenteron roof has nearly the same differentiation and inductive capacity before and after the invagination. Okada and Takaya (1942a, b), Okada and Hama ( 1943, 1944, 1945), and Hama (1949) demonstrated that the isolated uninvaginated dorsal blastoporal lip of a young gastrula of Triturus (representing presumptive prechordal endomesoderm) forms trunk-mesodermal structures and induces corresponding sections of the nervous system in competent ectoderm, whereas the same material after invagination differentiates into head mesenchyme and foregut endoderm and induces forebrain structures. The endo- and mesodermal portion of the archenteron roof situated in front of the notochordal anlage plays an important role in the spatial organization of the embryo, in that it is responsible for the formation of the head. Nieuwkoop, in Advances in Morphogenesis, 1973 D Differentiation Tendencies and Inductive Capacity of the Presumptive Prechordal Endomesoderm before and after its Invagination

Blastopore archenteron blastocoel