The round cluster becomes filled with fluid, and we have a hollow sphere of cells, which I call the blastula.
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.
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.
When the blastula is oval and free-swimming the inner mass is formed by unipolar immigration from the hinder pole.