Solid blastula is known as
A. Coeloblastula
B. Disco blastula
C. Stereo blastula
D. 5 – 13 days

Hint: The single-cell zygote undergoes multiple cycles of cleavage, or cell division, in order to create a cell ball, called a blastula, with a liquid-filled cavity in the centre of the zygote called a blastocoel. The blastula is typically a spherical layer (blastoderm) covering a fluid-filled or egg cavity (blastoderm).

Complete answer: The stereo blastula was observed in spiral-shaped eggs owing to the unavailability of blastocoel and is known in all molluscs excluding cephalopods. As the blastocoel isn't really there, it is termed as a solid blastula. In both of these, the origin of the blastocoel cavity exists in the blastula, however the tiny micromeres aggregate as a clump of cells across large, vegetative organized macromeres. At the completion of the cleavage, the outcome in the solid blastula designated stereo blastula. The cells mostly on the surface of the ball differentiate at a faster rate before the layer of the ball is surrounded by new cell layers. This platform works as an ectoderm, whereas the solid ball forms an endoderm in the centre, like in the previous shape of a stereo gastrula. Coeloblastula could also experience a sort of gastrulation which provides a solid gastrula called a stereo gastrula. During such a phase of development, the cells of the blastula split and relocate to the blastocoel, ultimately occupying the space. The migration of cells in this type of gastrulation is classified as ingression
So, the correct answer is option C.

Additional information: Coeloblastula is a void cell ball, single-cell thick. Blastula gastrulation of this type includes invagination, ingression, or delamination. Invagination entails a blastula flipping on itself, building a pocket with an opening. These are known as the archenteron and blastopore, and then they can become components of the gut. A disco blastula, unlike all the other types of blastula, will not build a cell ball across the original cell.

Note: Stereo blastula is a blastula which resides as a solid cluster of cells. The gastrulation in stereo blastula differs from that of gastrulation in the coeloblastula because there's no inner space for all the cells to differentiate into.