Ablastula is
a. a digestive organ
b. the innermost germ layer
c. a layer of embryological tissue
d. a hollow ball of cells in an embryo

d. a hollow ball of cells in an embryo

Explanation:

Blastula is a stage during early embryo development which arrives between morula stage and gastrula stage. This stage is characterized by a hollow ball of cells which surround a cavity known as blastocoele. This layer of cells which surround the blastocoele is known as blastoderm and it is formed by the repeated cleavages of zygote (fertilized egg). This cavity inside blastoderm i.e.  blastocoel is filled with fluid. Soon after the blastula develops fully, it turns into a gastrula and the process is called as gastrulation.

the law of original horizontality explains:   why rocks appear in layers. correct answer: b the law suggests that all rock layers are originally laid down (deposited) horizontally and can later be deformed.   the principle is used by geologists to make assumptions about the relative ages of sedimentary rocks.