Transformation of the solid morula (1) into the blastula (2), the first cavity that forms in the embryo.
The cavity is called the general cavity that forms in the embryo in the course of its development. It should not be confused with the embryonic digestive cavity (arquénteron), which is communicated with the outside by the blastopore (embryonic mouth) and therefore is not a general cavity in the embryonic sense.
After segmentation of the zygote, a solid sphere of cells called the morula is formed; it is almost always hollowed out to form a hollow blastula, so that all the cells are located on the periphery and leave a central cavity filled with liquid called blastocele, which is known as the primary general cavity, being the first to be formed in the embryo. This embryonic blastocele persists in the adults of animals called pseudocelomates, such as nematodes and rotifers.
However, the blastocele usually disappears in the next stages of embryonic development. Thus, in the flatworms, the mesoderm invades the blastocele that is practically obliterated, originating a massive body architecture, without a general cavity; these animals are called acellomates. On the contrary, in the embryos of the celomate animals (annelids, mollusks, arthropods, chordates), the mesoderm is hollowed and originates the coelom or general secondary cavity, so called because it is the second that is formed. The coelom may persist in the adult or may undergo different degrees of reduction and modification.
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