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Exercise 12 The 18-Hour Chick Embryo (Whole Mount) : Primitive He11se11 'S

1. The 18-hour chick embryo document describes the early stages of chick embryo development seen at 18 hours of incubation. 2. At this stage, the embryo resembles a straight line surrounded by dense masses and consists of structures like the primitive streak, primitive node, and primitive groove. 3. Students are asked to identify prominent features of the 18-hour embryo like the area pellucida, area opaca, primitive streak and its components, head process, and blood islands using prepared slides.

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Michaela Faye
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576 views2 pages

Exercise 12 The 18-Hour Chick Embryo (Whole Mount) : Primitive He11se11 'S

1. The 18-hour chick embryo document describes the early stages of chick embryo development seen at 18 hours of incubation. 2. At this stage, the embryo resembles a straight line surrounded by dense masses and consists of structures like the primitive streak, primitive node, and primitive groove. 3. Students are asked to identify prominent features of the 18-hour embryo like the area pellucida, area opaca, primitive streak and its components, head process, and blood islands using prepared slides.

Uploaded by

Michaela Faye
Copyright
© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
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Download as PDF, TXT or read online on Scribd
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' EXERCISE 12

THE 18-HOUR CHICK EMBRYO (WHOLE MOUNT)

Introduction

· The early stages of the chick embiyos are commonly identified by the number of hours of
incubati?n (at 38° - 39°q needed to reach the· point of development. Once the somites appeared,
th e number of somites may also be used.
At this stage, a definite embryo is starting to be formed and gradual unfolding of different
parts occurs from the germ layers to the primary organ rudiments until definitive organs have
developed . It, chick, the appearance of a primitive streak initiates the series of organogenesis.
The embryo resembles a straight line surrounded by dense staining masses.

Objectives
At the end of the exercise, the students should be able to:
I. identify the different structures found in the 18-hour chick embryo; and,
2. recognize the prominent features of the embryo.

Materials
'
CD of an I 8-hour chick embryo (whole mount)
3-D models of I 8-hour chick embryo
Prepared slides of an I 8-mm whole mount chick embryo

Procedure
I. View CD of an I 8-hour chick embryo. Take note of the most prominent features in this
stage.
2. Study3-D models of a chick embryo and compare the organs you have seen.
3. Get prepared slides of whole mount of 18-hr chick embryo. Using'LPO, scan the entire
embryo' and identify the following:

(a) Area Pellucida- clear, pear-shaped central region where the embryo is found.

(b) Area Opaca- blotchy and mottled region around the area pellucida.

(c) Primitive Streak- elongated mass of cells in the tenter of area pellucida. It is in the
process of regression. Its greatest length is attained at about the 19th hour of
, incubation. It consists of the following:

1. Primitive Node- a round mass of cells at the tip of the primitive streak. lt i'>
also called the Primitive Knot or the He11se11 's Node.

11. Primitive Pit- a small and clear area beside the primitive knot. This and the
primitive groove resulted from the sinking in of the cells.
111. Primitive Ridges or Folds- paired thickened " lines'· or dense mass of cells
that have migrated from the sides.

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' iv. Brimitive Groove- a long shallow canal that extend the full length of the
primitive streak. It is bounded on both sides by the primitive fold s.
v. Primitive Plate- the posterior end of the primitive streak seen as an area of
diffused cells.
I

(d) Head Process- dense mass of cells anterior to the Hensen 's node. It foreshadows
the notochord of the chick embryo.
(e) Proamnion- a transluscent area on the anterior region of the head fold .
What accoun(sfor its lucid appearance?

(f) Head Fold- a thick dark crescentic structure anterior to the neural plate. This may
not be seen in some whole mounts.
(g) Somites~ paired blocks of cells anterior to the primitive streak. These iater expand
, to enclose the developing neural tube. These can only be observed in embryos of
about 21 hours of incubation and onwards.
(h) Head ectoderm- a single layer of cells which delineate the head region and foregut
border.
(i) Notochord- tubular supporting structure seen as a faint thin line beneath the
primitive streak.
U) Blood Islands- a cluster of dense cells scattered in the area opaca vasculosa. They
are more concentrated lateral and caudal to the body of embryo.
(k) Area Opaca Vitellina- unmottled region beyond the area opaca vasculosa.

Label the parts of the whole mount of an 18-hour chick embryo.


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