Experimental investigation of the endoderm in early stages of development shows that the endodermal organ rudiments, like those derived from other germinal layers, are not initially determined, but that the endodermal cells destined to participate in the formation of the various organ rudiments are no more determined for their respective fates than are the cells of the other germinal layers. [...]
The somites, when first formed, are masses of mesodermal cells with a small cavity in the middle. The cells are arranged radially around the central cavity. With further development, the shape of the somites changes; they become extended in the dorsoventral direction and flattened mediolaterally. In the vertebrates with discoidal cleavage, this change in shape leads to the elevation of [...]
The relation between the archenteron and the definitive alimentary canal is very different in vertebrates with complete cleavage from that in vertebrates with incomplete (meroblastic) cleavage. The Relation in Lower Vertebrates: The archenteron in Amphioxus is originally lined with presumptive endoderm, presumptive mesoderm, and presumptive notochordal cells. When the notochord and the mesoderm are segregated from the endoderm, the latter [...]