Question

1. You conduct an experiment where you divide the chick blastoderm into four fragments.  Each fragment undergoes gastrulation and a new body axis is formed.  You also note that your intact control blastoderm only forms one axis.  What conclusions can you make from these results?

1. You conduct an experiment where you take a block of pre-somatic paraxial mesoderm and turn it up-side down.  The somites develop in the same order that they would have if they had remained in their original orientation.  What do these results suggest? What type of specification process is this?

Answer 1

Ever since Aristotle first followed its 3-week development, the domestic chicken has been a favorite organism for embryological studies. Moreover, at any particular temperature, its developmental stage can be accurately predicted.Fertilization of the chick egg occurs in the oviduct, before the albumen and the shell are secreted upon it. The egg is telolecithal (like that of the fish), with a small disc of cytoplasm sitting atop a large yolk. The first cleavage furrow appears centrally in the blastodisc, and other cleavages follow to create a single-layered blastoderm . As in the fish embryo, these cleavages do not extend into the yolky cytoplasm, so the early-cleavage cells are continuous with each other and with the yolk at their bases . Thereafter, equatorial and vertical cleavages divide the blastoderm into a tissue five to six cell layers thick. Between the blastoderm and the yolk is a space called the subgerminal cavity . This space is created when the blastoderm cells absorb fluid from the albumin and secrete it between themselves and the yolk. At this stage, the deep cells in the center of the blastoderm are shed and die, leaving behind a one-cell-thick area pellucida. This part of the blastoderm forms most of the actual embryo.

One of the major tasks of gastrulation is to create a mesodermal layer between the endoderm and the ectoderm.The formation of mesodermal and endodermal organs is not subsequent to neural tube formation, but occurs synchronously. The notochord extends beneath the neural tube from the base of the head into the tail. On either side of the neural tube lie thick bands of mesodermal cells. These bands of paraxial mesoderm are referred to as the segmental plate (in birds) and the unsegmented mesoderm (in mammals). As the primitive streak regresses and the neural folds begin to gather at the center of the embryo, the paraxial mesoderm separates into blocks of cells called somites. Although somites are transient structures, they are extremely important in organizing the segmental pattern of vertebrate embryos. As we saw in the preceding chapter, the somites determine the migration paths of neural crest cells and spinal nerve axons. Somites give rise to the cells that form the vertebrae and ribs, the dermis of the dorsal skin, the skeletal muscles of the back, and the skeletal muscles of the body wall and limbs.