vessels; others enter into the cavity, live in the fluid that fills it, and multiply by cleavage--the first blood-cells. But, besides these mesodermic cells of the "vascular layer" proper, other travelling cells, of which the origin and purport are still obscure, take part in the formation of blood in the meroblastic Vertebrates (especially fishes). The chief of these are those that Ruckert has most aptly denominated "merocytes." These "eating yelk-cells" are found in large numbers in the food-yelk of the Selachii, especially in the yelk-wall--the border zone of the germinal disk in which the embryonic vascular net is first developed. The nuclei of the merocytes become ten times as large as the ordinary cell-nucleus, and are distinguished by their strong capacity for taking colour, or their special richness in chromatin. Their protoplasmic body resembles the stellate cells of osseous tissue (astrocytes), and behaves just like a rhizopod (such as Gromia); it sends out numbers of stellate processes all round, which ramify and stretch into the surrounding food-yelk. These variable and very mobile processes, the pseudopodia of the merocytes, serve both for locomotion and for getting food; as in the real rhizopods, they surround the solid particles of food (granules and plates of yelk), and accumulate round their nucleus the food they have received and digested. Hence we may regard them both as eating-cells (phagocytes) and travelling-cells (planocytes). Their lively nucleus divides quickly and often repeatedly, so that a number of new nuclei are formed in a short time; as each fresh nucleus surrounds itself with a mantle of protoplasm, it provides a new cell for the construction of the embryo. Their origin is still much disputed. (FIGURE 2.361. Merocytes of a shark-embryo, rhizopod-like yelk-cells underneath the embryonic cavity (B). (From Ruckert.) z two embryonic cells, k nuclei of the merocytes, which wander about in the yelk and eat small yelk-plates (d), k smaller, more superficial, lighter nuclei, k apostrophe a deeper nucleus, in the act of cleavage, k asterisk chromatin-filled border-nucleus, freed from the surrounding yelk in order to show the numerous pseudopodia of the protoplasmic cell-body.) Half of the twelve stems of the animal world have no blood-vessels. They make their first appearance in the Vermalia. Their earliest source is the primary body-cavity, the simple space between the two primary germinal layers, which is either a relic of the segmentation-cavity, or is a subsequent formation. Amoeboid planocytes, which migrate from the entoderm and reach this fluid-filled primary cavity, live and multiply there, and form the first colourless blood-cells. We find the vascular system in this very simple form to-day in the Bryozoa, Rotatoria, Nematoda, and other lower Vermalia.