With the next process the formation of the young Myriapod may be said to begin. A strip along the length of the oval-shaped egg is thickened, and this thick mass of cells represents the future ventral surface of the animal. The rest of the thin layer of cells already mentioned just below the shell will form the shell or exoskeleton of the future animal. The thick strip of cells at the ventral surface has by this time split into layers, so that, resorting to our microscope again, a section through the short axis of the oval-shaped egg—a transverse section—will show us— 1. The egg-shell. 2. A layer of cells completely surrounding the egg, thin everywhere but on the ventral surface. This layer is known to embryologists as the _epiblast_. The thick part of the epiblast on the ventral surface gives rise to the nervous system. 3 and 4. Two layers of cells connected in the middle, along the line of the thick strip, but separate elsewhere, and not extending round the whole of the inside. These layers constitute what is known as the mesoblast, and give rise to the muscles and most of the internal organs. 5. The scattered cells in the yolk. They are known as the hypoblast and give rise to the digestive canal. After this point is reached the formation of the organs begins. The segments are formed in order from before backwards. First the head, then the next segment, and so on. When the number of segments with which the animal will be hatched are formed, another process begins, and the tail end of the animal, which can already be distinguished, is bent towards the head. This is a process that takes place in many animals besides Myriapods, and is called the formation of the ventral flexure. Shortly after this the animal bursts the shell and comes {66}into the outer world. The various processes may be understood by reference to the Figs. 36, 37, 38, 39, which are successive stages in the development of a Chilognath. Figs. 37, 38, are thin slices through the shorter diameter of the egg, which, as before mentioned, is an oval in shape. Fig. 39 is a section through the longer diameter of an egg in a more advanced stage of development, in fact just about to burst the shell. The body of the future animal is marked by constrictions, the future segments. Some of the organs are already formed, as the brain and the digestive tube, the openings of which will form the mouth (_st_) and the anus (_pr_). [Illustration: FIG. 38.—Transverse section through next stage: _mk_, keel-like mass of cells from which the mesoblast is produced; _ec_, epiblast. (From Heathcote, Post. Emb. Dev. of _Julus terrestris_; _Phil. Trans._ vol. 179, 1888, B.)] [Illustration: FIG. 39.—Longitudinal section through later stage: _Segs._ 2, 3, etc., segments; _Ceph. Seg_, head; _mes_, mesoblast; _en_, hypoblast; _st_, future mouth; _pr_, future anus; _mesen_, gut; _mem.ex_, as in Fig. 41. (From Heathcote, Post. Emb. Dev. of _Julus terrestris_.)]