The next obvious change is that the cells at the lower portion of the mass which projects into the cavity appear to get flattened out--at any rate they obviously arrange themselves in a definite and separate layer; and this layer in its turn proceeds to go on growing by division of its cells in such a way as to form another little closed cavity within the larger one. This cavity is termed the "yolk sac." (See Fig. 3.) Then another little cavity occurs, this time within the original projecting cell-mass. This cavity is termed by embryologists the "amniotic cavity," and the cells which line it, and which in their turn become arranged as a separate layer, form what is termed the "embryonic ectoderm." (See Fig. 3.) [Illustration: FIG. 3.] It is in this region, and in that of the yolk sac which lies just underneath it, that the future growth of the embryo itself occurs, and the portion is therefore termed the "embryonic area." (See Fig. 3.) Up to this point we have seen that two layers of cells have appeared, one round the yolk sac, called the "entoderm," and the other lining the amnion, called the "ectoderm." After these two germinal layers have made their appearance, a third layer comes into existence, which, because it begins growing from the embryonic area, and lies between the two already mentioned, has received the name of the "mesoderm." This third germinal layer divides into two portions before very long, and the space between these two is that in which the body cavity itself subsequently arises. One part of the mesoderm, situated near one end of the embryonic area, is specially important, because in it are formed the blood-vessels which supply the embryo, and which ultimately afterwards becomes the "umbilical cord," which forms the connection between embryo and mother. CHAPTER VI EARLY DEVELOPMENT The early development of the embryo now proceeds rapidly, and its appearance at the stage we have just been describing is thus stated by Dr. R. W. Johnstone:-- "If the ovum at this stage be looked at from above, the embryonic area appears as a small shaded oval. The shading is due to an increased growth of cells, because here the three germinal layers--embryonic ectoderm, mesoderm, and entoderm--are in contact. At one end a patch of darker shading indicates a still greater growth of cells. Running forward from this is a band--the _primitive streak_--in the centre of which lies a darker line--the _primitive groove_. At the far (anterior) end of the primitive groove there is a dark spot--Hensen's node--from which still another streak runs forward, the head process. Later, in front of the primitive streak, a thickened band of ectoderm appears, broadening out posteriorly. The edges of this band rise up to form two folds, which meet anteriorly. The groove between them is the _medullary groove_, and ultimately they fold over and unite to form the _neural canal_. (See Fig. 4.)