Blastosphere

Blastosphere, or Blastula. After the fertilisation of an ovum or egg it commences development by dividing into two; each half again divides, and these parts continue to sub-divide into 8, 16, 32, and so on, till the ovum is composed of a mass of a large number of cells. In this stage it is called a "morula." and each of these cells is a blastomere. In most cases these blastomeres arrange themselves in a single layer called the blastoderm, forming a spherical shell enclosing a central cavity. In this stage it is a blastosphere, and the cavity in it is the blastoele, and it usually opens to the exterior by an aperture known as the blastopore. In some cases this pore may persist through life as either the mouth or anus of the adult, but in most cases it closes and the permanent openings form elsewhere. In some rare cases the blastocoele may remain as the body cavity of the adult.