![]() At this magnification the complete yolk must be imagined as about three feet in diameter. Only the blastoderm and the immediately underlying yolk are included in the diagram. With the establishment of the blastocoele the embryo is said to have progressed from the morula to the blastula stage.įigure 7, D, shows the conditions seen on sectioning the blastula of a bird. The marginal area of the blastoderm in which the cells remain undetached from the yolk and closely adherent to it, is called the zone of junction. The space thus established between the blastoderm and the yolk is termed the segmentation cavity (blastocoele). A cavity is formed beneath the blastoderm by the detachment of its central cells from the underlying yolk while ^ the peripheral cells remain attached. Almost as soon as it is established there begins a rearrangement of the cells presaging the formation of the blastula. The morula condition is of short duration. and completely defined at the periphery the cells are flattened, larger in surface extent, and are not walled off from the yolk beneath. In the center of the blastoderm the cells are smaller. It consists of a discshaped mass of cells several strata in thickness, the blastoderm, lying closely applied to the yolk. At the end of segmentation the chick embryo has arrived at a stage which corresponds with the morula stage of forms with less yolk. This is known as a morula from its resemblance to the mulberry fruit which is in form much like the more familiar raspberry or blackberry. In eggs without a large amount of yolk, segmentation results in the formation of a rounded, closely packed mass of blastomeres. The nomenclature and limitation of the various phases of development are largely arbitrary and the use of terms designating phases or stages of development should not be allowed to obscure the fact that the whole process is a continuous one. Development progresses from phase to phase without abrupt change or interruption. Segmentation is regarded as ending when the progress of development ceases to be indicated merely by increases in the number of cells, and begins to involve localized aggregation and differentiation of various groups of cells. The end of the segmentation stage is not marked by even a retardation in the succession of mitoses. ![]() It should by no means be inferred that cell division ceases with the cleavage divisions. The Establishment of the Entoderm The Morula Stage Historic Disclaimer - information about historic embryology pages The Early Embryology of the Chick 1920: Introduction | Gametes and Fertilization | Segmentation | Entoderm | Primitive Streak and Mesoderm | Primitive Streak to Somites | 24 Hours | 24 to 33 Hours | 33 to 39 Hours | 40 to 50 Hours | Extra-embryonic Membranes | 50 to 55 Hours | Day 3 to 4 | References | Figures Those interested in historic chicken development should also see the earlier text The Elements of Embryology (1883).įoster M. Developmental defects at the foramen ovale. My thanks to the Internet Archive for making the original scanned book available.īy the same author: Patten BM. Currently only the text has been made available online, figures will be added at a later date. There may also be errors in transcription or interpretation from the original text. As with all historic texts, terminology and developmental descriptions may differ from our current understanding. If like me you are interested in development, then these historic embryology textbooks are fascinating in the detail and interpretation of embryology at that given point in time. This historic 1920 paper by Bradley Patten described the understanding of chicken development.
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