gastrula

The embryo in the stage of development following the blastula; in lower forms with minimal yolk, it is a simple double-layered structure consisting of ectoderm and endoderm enclosing the archenteron, which opens to the outside by way of the blastopore; in forms with considerable yolk, the configuration of the g. is greatly modified owing to the persistence of the yolk throughout the gastrulation process. SYN: invaginate planula. [Mod. L. dim. of G. gaster, belly]

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gas·tru·la 'gas-trə-lə n, pl -las or -lae -.lē, -.lī an early metazoan embryo in which the ectoderm, mesoderm, and endoderm are established either by invagination of the blastula (as in fish and amphibians) to form a multilayered cellular cup with a blastopore opening into the archenteron or (as in reptiles, birds, and mammals) by differentiation of the upper layer of the blastodisc into the ectoderm and the lower layer into the endoderm and by the inward migration of cells through the primitive streak to form the mesoderm compare MORULA

gas·tru·lar -lər adj

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n.

an early stage in the development of many animal embryos. It consists of a double-layered ball of cells formed by invagination and movement of cells in the preceding single-layered stage (blastula) in the process of gastrulation. It contains a central cavity, the archenteron, which opens through the blastopore to the outside. True gastrulation only occurs in the embryos of amphibians and certain fish, but a similar process occurs in the embryonic disc in other vertebrates, including humans.

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gas·tru·la (gasґtroo-lə) the early embryo in the stage following the blastula or blastocyst. The simplest type consists of two layers, the ectoderm and the endoderm, and of two cavities, one lying between the ectoderm and the endoderm; the other (the archenteron) is formed by invagination so as to lie within the endoderm and having an opening (the blastopore). In human embryos the gastrula stage occurs during the third week, as the embryonic disc becomes trilaminar, establishing the ectoderm, mesoderm, and endoderm from the epiblast.