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Observation
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Observation
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Part 2. Recognition of Later Embryonic Developmental Stages
In this part of the exercise, you will be given 6 embryo samples at different stages of development. Your task will be to arrange the samples in the proper order from youngest to oldest, then to determine the most probable age, in hours, of each sample. Use the compound scope for these observations.
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You will use Table 1 (Early Stages of Development) as the source of information needed to complete Part 2 of this exercise.
A. Notochord - a retractile rod passing along the axis. The cells are giant and have a vacuole containing a cartilage-like material that gives the notochord strength. At the younger stages (1 8-24 h) you will be able to see developmental progression of cell differentiation in the tail of the embryo.
B. The dorsal tubular nervous system - The spinal chord runs the length of the body just dorsal of the notochord. Its tubular structure is easily visualized as a simple hollow epithelium in the tail.
Pharyngula Period (24-48 h)
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| prim-6 25 h |
high pec 42 h |
The heart is first visible as a simple straight tube just posterior to the anterior limit of the eye. It begins to beat at 24 h with no apparent direction to the beat. By 26 h, the contraction occurs as an Apgoing wave.
Blood cells begin to collect in the region between the yolk and the notochord, where the major vessels supplying the trunk and tail will form. Shortly after blood becomes visible on the yolk, circulation begins.
Pigmentation formation begins, both in cells in the margin of the retinal pigment layer, and in neural crest derived melanocytes found dorsolaterally in the skin.
At about 30 h, you can see blood slowly circulating through the single pair of aortic arches that will become the most anterior ones of the eventual series of five pairs.
At 30 h, a light touch placed anywhere on the body or head usually elicits a robust reflexive side-to-side bout of wiggles.
48 h embryo
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a. eye - note pigmentation in the retina
b. heart - just posterior of eye on ventral surface of yolk sac
c. ear - easily visible just dorsal and posterior of the eye
d. gill arches - just ventral of the ear above yolk sac
e. notochord - easily visible in center of somites
f. spinal cord - look just dorsal of notochord and slightly move focus up and down
g. hatching gland cells - noticeable over the anterioventral surface of the yolk sac
Interesting observations:
a. blood circulation - Observe how blood leaves the heart and passes into the aortic arches where at later stages it will be aerated via the gills. At these early stages, aeration occurs on the yolk sac. Follow the blood from the heart to the tail and back.
b. muscle cells - elongated striated skeletal muscle cells can be seen stretching horizontally across the individual somites from one segmental border to the next
c. melanocytes - easily viewed in tail area
