Blastulas [noun]

The hollow sphere which is thus formed is the important stage of the “germinal vesicle,” the blastula, or blastosphere.

Consider its mode of division, and the formation of the blastula, gastrula, and germinal layers.

In a blastula showing complete segmentation the blastomeres of the upper hemisphere are the more finely subdivided.

Our next question concerns the distribution of potentiality, when the embryo is developed further than the blastula stage.

But there is in the blastula no trace of one part of the germ becoming different with respect to others of its parts.

And a similar argument might apply to the small but whole blastula of Echinus, and to all other cases.

This blastula is formed of superposed layers, each of which gives birth to specialised organs in the embryo.

The blastula differentiates itself into embryonic layers, the ectoderm, endoderm, and mesoderm already mentioned.

In the third stage the Inner Cell-Mass separates into two layers derived from the inner cell-plate of the blastula.

By the conversion of the one-layered blastula into two layers of cells, the Gastrula stage of the embryo is attained.