The hollow sphere which is thus formed is the important stage of the “germinal vesicle,” the blastula, or blastosphere.
The round cluster becomes filled with fluid, and we have a hollow sphere of cells, which I call the blastula.
But there is in the blastula no trace of one part of the germ becoming different with respect to others of its parts.
In a blastula showing complete segmentation the blastomeres of the upper hemisphere are the more finely subdivided.
Consider its mode of division, and the formation of the blastula, gastrula, and germinal layers.
Our next question concerns the distribution of potentiality, when the embryo is developed further than the blastula stage.
In the third stage the Inner Cell-Mass separates into two layers derived from the inner cell-plate of the blastula.
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.
When the blastula is oval and free-swimming the inner mass is formed by unipolar immigration from the hinder pole.
blastula blas·tu·la (blās'chə-lə)
n. pl. blas·tu·las or blas·tu·lae (-lē')
An early embryonic form produced by cleavage of a fertilized ovum and consisting of a spherical layer of cells surrounding a fluid-filled cavity.
The stage of an embryo that consists of just over a hundred cells — a stage reached about one week after fertilization. At this stage the cells are just at the very beginning of cellular differentiation and are said to be totipotent (See totipotency).
Note: This is the stage of development where embryonic stems cells can be harvested for medical research.