VAN 211: Veterinary Histology and Embryology (2+2)

GASTRULATION IN AMPHIOXUS

• The animal pole of the blastula corresponds roughly to the front end of the future embryo. The animal and vegetal poles are the future ectoderm and endoderm respectively. In between, is a girdling zone, which is subdivided into prospective mesoderm, notochord and neural plate.
• Invagination of vegetal cells is followed by involution of cells, around the margin of the double walled cup, thus being formed. The continuation of these movements carries endoderm, mesoderm and notochord to the interior and obliterates the original blastocoel.
• The central cavity now formed is archenteron and its mouth at the vegetal pole is blastopore. At this period, the young embryo is termed a gasturla.
• The margin around which involution took place is the lip of the blastopore. Backward growth of this lip-region, lengthens the embryo and upward growth of the ventral lip elevates the blastopore, thus tilting up the mesodermal portion.
• Now, the roof of the archenteron consists of a median strip of notochordal cells, flanked on each side by a strip of mesodermal cells (which were tilted up by the elevation of the ventral lip of the blastopore). The entoderm then closes the dorsal defect caused by the loss of notochord and other mesoderm and thus produces the definitive, tubular gut.
• The cells left on the external face on the gastrula form the ectoderm. Dorsally they constitute the neural plate while the rest of the area forms the general covering of the embryo, which will become epidermis.