Heredity and Development Second Edition (1972) / Chapter Skim
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11 Gastrulation and Organ Formation
11 Gastrulation and Organ Formation
Pages 242-255
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From page 242... ...
Yolk granules are denser than ordinary cytoplasm and, as a result, the vegetal hemisphere, with its abundant supply of yolk granules, is heavier than the animal hemisphere. As a consequence of this density difference the animal hemisphere is uppermost throughout the cleavage and blastula stages.
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From page 243... ...
The vegetal hemisphere cells invaginate through the blastopore and the animal hemisphere cells spread over the entire embryo to form its outer covering. At the end of gastrulation, the cells frequently become arranged in three concentric layers, which from the outside to the center are the ectoderm (‘outer skin')
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From page 244... ...
Embryo a is an early gastrula with the dorsal lip of the blastopore just formed; b is a middle gastrula with a large yolk plug; c is a late gastrula with a small yolk plug. The eight colored areas undergo extensive movements during gastrulation.
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From page 245... ...
The presumptive endoderm is restricted to the ventral portion of the vegetal hemisphere. This area will form the lining of the alimentary canal and structures derived from it, such as the liver, pancreas, and bladder.
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From page 246... ...
b is a view toward the dorsal lip (modified from Roux Arch.
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From page 247... ...
During gastrulation one should pay special attention to the movements of the presumptive notochord and the presumptive neural tube areas. Gastrulation begins with the formation of the dorsal lip of the blastopore at about 22 hours after fertilization.
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From page 248... ...
The presumptive neural tube and presumptive notochord areas are expanding to cover larger portions of the surface. Four hours later, the gastrula shows important changes (Fig.
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From page 249... ...
Longitudinal section and diagram of the presumptive regions. the presumptive notochord cells have invaginated.
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From page 250... ...
Earlier, the presumptive notochord formed the archenteron roof, but by the late neurula stage the endoderm has moved up from the sides and formed a layer beneath the notochord. Some of the details of neural tube formation can be illustrated better
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From page 251... ...
Beneath the ectoderm there is a continuous layer of mesoderm, which forms the notochord on the dorsal mid-line. The remaining portion of the mesoderm is a thin layer, which is difficult to see in the photograph, but is recognizable by the numerous darkly stained nuclei.
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From page 252... ...
Apart from the closure of the neural folds, this stage is not much different from the preceding one. The embryo consists of the three embryonic layers with little cell differentiation.
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From page 253... ...
Late neurula. Cross section.
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From page 254... ...
The optic cups will form the retina, which is the portion of the eye that is sensitive to light. The epidermis adjacent to the optic cup forms the lens.
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From page 255... ...
This brief survey of early development in the amphibian embryo was designed to provide a background for the consideration of the problems of embryology. Now that we have learned something of how an embryo develops we can consider some of the controlling processes that are responsible for embryonic differentiation.
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