. The development of the human body : a manual of human embryology. Embryology; Embryo, Non-Mammalian. 56 FORMATION OF THE GERM LAYERS a relatively small dark area on the surface, which is the embryonic disk. But if it be looked at from the side (Fig. 30, A), it will be seen that the upper half of the ovum, that half in which the embryonic disk occurs, is somewhat darker than the lower half, the line of separation of the two shades corresponding with the edge of the primary endoderm which has extended so far in its growth around the inner surface of the enveloping layer. A little later a dark
. The development of the human body : a manual of human embryology. Embryology; Embryo, Non-Mammalian. 56 FORMATION OF THE GERM LAYERS a relatively small dark area on the surface, which is the embryonic disk. But if it be looked at from the side (Fig. 30, A), it will be seen that the upper half of the ovum, that half in which the embryonic disk occurs, is somewhat darker than the lower half, the line of separation of the two shades corresponding with the edge of the primary endoderm which has extended so far in its growth around the inner surface of the enveloping layer. A little later a dark area appears at one end of the embryonic disk, produced by a prolifera- tion of cells in this region and having a somewhat crescentic form. As the embryonic disk increases in size a longitudinal band makes its appearance, extending forward in the median line nearly to the center of the disk, and represents the primitive streak (Fig. 30, B), a slight groove along its median line forming what is termed the primitive groove. In slightly later stages an especially dark spot may be seen at the front end of the primitive streak and is termed Hensen's node (Fig. 30, C, hn), while still later a dark streak may be observed extending forward from this in the median line and is termed the head-process of the primitive Fig. 31.—Posterior Portion of a Longitudinal Section through the Embryonic Disk of a Mole. bl, Blastopore, ec, ectoderm; en, endoderm; prm, prostomial mesoderm.—{After Heape.) To understand the meaning of these various dark areas recourse must be had to the study of sections. A longitudinal section through the embryonic disk of a mole ovum at the time when the crescentic area makes its appearance is shown in Fig. 31. Here there is to be seen near the hinder edge of the disk what is potentially an opening (bl), in front of which the ectoderm (ec) and primary endoderm (en) can be clearly distinguished, while behind it no such distinction of. Please note that these
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