THE PRIMITIVE GROOVE. Embryonic growth be gins by the formation of a long depression in the middle of the germ-disk, called 'primitive groove' (Fig. 15), which indicates the region where a mod ified gastrulation is beginning; also at the hinder edge of this disk a depression is seen, and cells pass forward between the ectoderm and yolk mass, and others are budded off from the yolk to form the ectoderm. As the embryo grows longer the posterior gastrula-depression retreats back ward and the median depression elongates. Al most simultaneously, from the region of the primitive groove, the mesoderm is produced to the right and to the left. A little later a rod of cells is cut off below the primitive groove, mak ing the notochord. around which the vertebral column will later arise. At about IS hours a pair of folds rise up at the right and left of the primitive groove. These are the medullary folds, and the depression between them consti tutes the medullary furrow. The folds grow higher, and arch over toward each other until a complete tube—the medullary or neural tube —is formed. This gives rise to the brain and spinal cord. By the middle of the second day of ineubation the head of the chick is marked off by the enlarging brain, the trunk is com pleted at the tail end, and the mesoderm is seen to be composed of a series of paired blocks of tissue; these correspond to the metameric seg ments of the body, and arc called primitive somites. From them the muscles of the body
wall will arise. Laterally, the mesoderm exists as two thin plates or layers, one of which— the somatic layer lies (dose to the ectoderm, and the oilier—I he visceral or splanehnic layer— lies next the forming food-eanal. Between these layers lies the body-eavity.
The embryo is now well outlined. The food canal has a roof, but its floor is a mass of yolk which the ectoderm has gradually enveloped be low as well as above. The embryo is gradually raised up above the great mass of the yolk, which now appears as a sac pendent from the under side of the nascent. intestine. The head rapidly enlarges, month and eyes appear, mesenehyma tons cells are laid 111)\\ II around the notochord and form the bones of the vertebral column; the nerves grow forth from the walls of the neural lobe; kidney organs. heart, and appendages of the food-tube arise in rapid succession I Figs. In•