General features of animals

The following features characterize members of the Kingdom Animalia:

(h) Embryonic Development (morula blastula gastrula)
Most animals have a similar pattern of embryonic development. The zygote first undergoes a series of mitotic divisions, called cleavage, and like this dividing frog egg, becomes a solid ball of cells, the morula, and then a hollow ball of cells, the blastula. In most animals, the blastula folds inward at one point to form a hollow sac with an opening at one end called the blastopore. An embryo at this stage is called a gastrula. The subsequent growth and movement of the cells of the gastrula differ widely from one phylum of animals to another.

Unique Tissues. The cells of all animals except sponges are organized into structural and functional ul1its called tissues, collections of cells that have joined together and are specialized to perform a specific function. Animals are unique in having two tissues associated with movement: (1) muscle tissue, which powers animal movement, and (2) nervous tissue, which conducts signals among cells.