In this article we will discuss about the gastrulation and formation of primary organs rudiments in birds. The earlier stages of the discoidal cleavage in the eggs of reptiles and birds resemble essentially the cleavage in meroblastic eggs of the fishes. The blastoderm is several cell layers thick, and lies over a "sub-germinal cavity." At a later stage of development [...]
In this article we will discuss about the gastrulation and formation of primary organ rudiments in fishes. In fishes the yolk formed in the oocytes during oogenesis initially does not differ substantially from the yolk in amphibian oocytes and eggs. It consists of yolk platelets lying in the cytoplasm and intermingled with other cytoplasmic components (mitochondria and the like). The [...]
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 [...]