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We call this layer the blastoderm, and the sphere itself the blastula, or embryonic vesicle. This interesting blastula is very important. The conversion of the morula into a hollow ball proceeds on the same lines originally in the most diverse stems as, for instance, in many of the zoophytes and worms, the ascidia, many of the echinoderms and molluscs, and in the amphioxus.

Livingstone as he stooped to pick up a specimen of the wild fruit morula; but she strangely stopped stock-still when less than her own length distant, and gave him time to escape; a branch pulled out his watch as he ran, and turning half round to grasp it, he got a distant glance of her and her calf still standing on the selfsame spot, as if arrested in the middle of her charge by an unseen hand.

In certain animals belonging to every one of the chief groups into which the Metazoa are divisible, the cells of the cell-aggregate which results from the process of yelk-division, and which is termed a morula, diverge from one another in such a manner as to give rise to a central space, around which they dispose themselves as a coat or envelope; and thus the morula becomes a vesicle filled with fluid, the planula.

They belong to various groups both of the Protophyta and Protozoa. To have some idea of those ancestors of our race that succeeded phylogenetically to the Moraeada, we have only to follow the further embryonic development of the morula. In this way the solid mulberry-embryo becomes a hollow sphere, the wall of which is composed of a single layer of cells.

Inside the membrane that surrounds the ovum, the stem-cell of the Ascidia, after fecundation, passes through just the same metamorphoses as the stem-cell of the Amphioxus. It undergoes total segmentation; it divides into two, four, eight, sixteen, thirty-two cells, and so on. By continued total cleavage the morula, or mulberry-shaped cluster of cells, is formed.

It is clear that this morula reproduces for us to-day the simple structure of the multicellular animal that succeeded the unicellular amoeboid form in the early Laurentian period. In accordance with the biogenetic law, the morula recalls the ancestral form of the Moraea, or simple colony of Protozoa.

In this way we get the spherical, mulberry-shaped body, which we call the morula. The segmentation of the Amphioxus is not entirely regular, as was supposed after the first observations of Kowalevsky . It is not completely equal, but a little unequal.