Page:The New International Encyclopædia 1st ed. v. 07.djvu/42

* EMBRYOLOGY. 28 longitudinal cleavages into a more or less ovoidal body — the embryo. Certain of the cells of the embryo may early become specialized to form a thread composed of a single row of cells and EMBRYOLOGY. am emb ar.vasc FlG. 16. ADVANCED EMBRYO CHICK. Condition on fifth day: s, shell; a, air-chamber; emb, em- bryo, lying on the left side, with the head up; am, amnion; all, allantois, in the cavity between the amniotic layers, and performing the chief respiratory function; ar.vasc, vascular area (vitelline blood) gradually extending their ramifications over the yolk-sac m/s), and absorbing its nu- triment. called the suspensor. By means of the suspensor the embryo is pushed out into the nutritive Quids of the embryo-sac. As cell-division pro- ceeds a peripheral layer of cells is differentiated, which is the epidermis, also an axial column of cell- is often early discernible, with the root-cap cell or cell, at the end next to the suspensor, and 1 ii. stem-tip a1 the opposite end. The cotyledons cell-multiplication. In some cases the plumule with its nascent leaves is already formed from t P i IIDR1 i'IIIck. gac; f '■■ . b.br, bind brain; ft./, hind limb; / i xn.br, midbrain; mn, man- dlbuiai n Parker and Etaswell.) i mi l ip before germinal inn. 'II, contain the embryo as ■< minute germ iml i, ed ii ""i i lie embi and |< e oi ule ( pepper) or tl" i bed by the - .in. I be lodged m t lit- cotj ledons l ii i i opmi i '-i v Clin K. The hi i In ihi il .m.l albumen laid down around the egg proper, or 'yellow af the egg,' as it passes along the maternal oviduct. The yellow of the egg 1 is usually while it is still in the oviduct of the hen only a single cell, which is immensely dis- tended by food-materials or food-yolk. The nucleus lies in a thin sheet of protoplasm on top of the 'yellow.' This nucleus divides, but the whole yolk does not, so that partial cleavage oc- curs, producing a disk of several cells at the time the egg is laid. This disk constitutes the cicatrix, or 'scar' or 'tread' of the egg, which can be readily seen with the naked eye. It is technically called the germ-disk, or blastoderm, and consists of an upper and an under layer. Later a middle layer of cells arises, and the blastoderm is differentiated into a central trans- parent area pellucida, on which the embryo forms, and a peripheral area opaca, which is continuous with the central area and extends laterally until it sourrounds the entire volk (Figs. 1-4). 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 entoderm. As the embryo grows longer the posterior gastrula-depression retreats hack- 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 18 hours a pair of folds rise up at the right and left of i In 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 tub< — the medullary or neural tube —is formed. Tin- gives rise to the brain and spinal cord. By the middle of the second day of incubation the head of the chick is marked off by the enlarging brain, the trunk is com- pleted ,it 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 are called primitive iie-. From them the muscles of the body- wall »ill arise. Laterally the mesoderm exists i two thin plates or layers, one of which — the somatic layer lies olose to the, ectoderm, and the othei the visceral or splanchnic layer — lies nest the forming food-canal. Between these layers lies the bodj eai Ity. The embryo i now well outlined. The food- ■ .i na I iii -i roof, but it Boor i- o mass of yolk which the ectoderm has gradually enveloped be- ,11 as above. The embryo is graduallj raised up above the great mass of the yolk, which mm appt .i il from the under -ill ill. n. I intestine, The head rapidly enlarges, mouth and eyes appear, mesenchyma- i cells are laid lund the notochord form the b be vei tebral column ; the spinal ii. m grow forth from the walls of the neural tube; kidnej organs, heart, Id I vessels, and appendagi - of the i i ube ari le in rapid I igs, oil: 16; 17).
 * ■ i « early formed by a pair of lateral regions of