Primary Streak

PRIMARY STREAK, or primary band, represents a section of the avian embryonic disc or mammalian embryonic shield (see Embryo), from which the formation of the most important embryonic rudiments—mesoderm, dorsal cord (notochord), and neural plate—occurs. The primary streak appears at the beginning of the so-called second phase of gastrulation after the formation of the outer and inner embryonic layers. The primary streak was first noted by Baer (K. E. v. Baer; 1828-34) in the chicken embryo and was then studied in detail both in bird embryos (most thoroughly in the chicken) and in mammalian embryos (primarily rabbit and dog). The significance of the primary streak as a greatly modified blastopore (see) was formulated by O. Hertwig (O. Hertwig; 1881) on the basis of the comparative-embryological method. In the chicken embryo, the primary streak appears on the 8th-14th hour of incubation in the posterior part of the so-called light field along the median line. In mammals, the primary streak is visible as a distinctly delimited line located in the posterior half of the embryonic shield. Anteriorly, the primary streak has a club-shaped thickening, the so-called primary, or Hensen's, node. As development continues, the primary streak increases in length in the posterior direction, its end coinciding with the posterior edge of the embryonic shield. Along it appears a longitudinal groove, bordered by protruding, so-called primary folds. A cross-section of the embryo at this stage shows that the primary streak represents a thickened area of ectoderm, formed due to the enhanced multiplication of its cellular elements. On both sides of this thickening, a paired plate begins to grow, gradually advancing between the ectoderm and endoderm, the so-called peristomal mesoderm. In the area of Hensen's node, the primary streak fuses with the endoderm, and a cross-section here reveals a depression or primary pit. Posteriorly, the primary groove becomes less deep and disappears completely at the posterior end of the primary streak. Anteriorly, from the primary node, a head process—a cellular cord fused with the endoderm and separated from the ectoderm by a slit-like space—gradually increases in length. After reaching its maximum development, the primary streak then gradually shortens, retreating in the caudal direction, 24 since the elongation of the head process occurs at the expense of its anterior end. In some cases in mammals (e.g., in humans, in bats), there is a so-called chordal canal in the thickness of the head process, communicating with the primary pit of Hensen's node. A rudimentary canal, comparable to the chordal canal of mammals, is also observed in some cases in birds. On both sides of the head process, the so-called parachordal mesoderm plates grow, which are a continuation forward of the aforementioned peristomal mesoderm. Subsequently, the head process becomes isolated from both the endoderm and mesoderm and transforms into the notochord. The section of ectoderm located anterior to the primary node, above the head process, gives rise to the neural plate, while the anterior part of the primary groove, after the closure of the neural tube, transforms into the so-called neurenteric canal (canalis neurentericus). The significance of the primary streak in the developmental process is clarified by data from comparative embryology and developmental mechanics. From the point of view of the blastopore theory developed by Hertwig, the primary streak and primary groove can be compared in their development with the processes of embryonic development observed in a simpler form in the lancelet, amphibians, and fish. The gradual shift of the primary streak posteriorly and the formation anterior to it of the rudiment of the notochord (from the head process) and the rudiment of the nervous system (from the ectoderm) correspond in lower vertebrates and in the lancelet to the transformations that take place in the area of the so-called dorsal lip of the blastopore. In the process of phylogenesis, the closure of the primary mouth or blastopore has undergone significant changes; in higher vertebrates, this process occurs already in a much masked form. Data from developmental mechanics (Spemann) show that the dorsal lip of the blastopore represents an area of the embryo that has an exceptional influence on the development of the entire so-called axial complex of the embryo (notochord, spinal cord, somites). The experimental destruction of the dorsal lip of the blastopore leads to a cessation of development of the entire axial complex. Experiments on the chicken embryo showed that the primary streak, specifically its thickened part, the so-called Hensen's node, has the same significance. Peebles showed that the destruction of Hensen's node at the 18-hour incubation stage completely delays embryonic development. If, however, the primary streak is destroyed posterior to the node, leaving the latter intact, then the axial complex develops except for a few posterior segments. Thus, on the basis of Peebles' experiments, further developed by Wetzel, the primary, or Hensen's, node is an organizer in the sense of Spemann (see Developmental Mechanics). n. hlopin.