The changing appearance of the embryos, especially during organogenesis, invites the distinguishing of certain stages which can be referred to when it is desired to indicate how far an embryo has progressed in its development. Tables of “normal stages” have been worked out for a number of species of animals, especially those that are most often used for research.
In the latter half of the nineteenth century, an ambitious project on establishing series of normal stages for a large number of animals was undertaken by Keibel and his collaborators.
This work was an essential contribution to the science of comparative embryology but went into oblivion later, when the interest of the great majority of embryologists shifted from a descriptive to an experimental approach to the development of animals.
However, it soon became evident that tables of normal stages were quite as important for experimental work, as it was often necessary to indicate precisely at what stage an operation or other experiment was carried out.
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The table of normal stages of Ambystoma (= Amblystoma) punctatum, prepared by Harrison, was the first made expressly with experimental investigations in mind. Harrison’s table was published posthumously, though earlier it was made accessible privately to many workers in the field and has been widely used. Harrison’s stages were also redrawn and included in Rugh’s book, Experimental Embryology (1948).
Other tables of normal stages followed; the most widely used ones are probably the stages of Rana pipiens by Shumway (1940) and by Rugh (1948), the stages of the chick by Hamburger and Hamilton (1951) and the stages of Xenopus laevis by Nieuwkoop and Faber (1956). There is still no complete table of normal stages of the development of the human embryo.
In compiling a series of normal stages the embryologist is confronted with the task of deciding what characteristics should be selected for distinguishing one stage from another.
The characteristics that are often used are:
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i. Age of the embryo.
ii. Size of the embryo.
iii. Morphological peculiarities of the embryo.
In human embryology particularly, the first two criteria are often used. It is customary for an author to refer to embryos by age (five-week embryo, two-month-old embryo) or by size (an embryo or fetus of so many millimeters crown-rump length). Both these criteria are, however, not very convenient.
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The age of an embryo is often not known, and in animals other than mammals, the rate of development is dependent on the temperature of the environment to such an extent that a statement about the age of the embryo is meaningless unless the temperature at which the development has proceeded is likewise indicated.
The size of the embryo is no true indication of its degree of development, as the dimensions of the embryo vary to a great extent. Moreover, some variability in the size of embryos may be resolved later in the course of development, which adds to the difficulty of using size as a criterion for the definition of stages.
What remains is to base the normal stages on morphological properties of the embryo and especially on properties that can be easily ascertained by external examination of the embryo, without its fixation or dissection—that is, identification based largely on external features.
In the initial stages of development (cleavage stages), the number and size of the blastomeres may conveniently be used. During gastrulation, the shape of the blastopore or its equivalent (primitive streak) may be used, and just after gastrulation, the neural plate offers easily recognizable features.
During early organogenesis, the number of pairs of somites has often been used to define the stage of development of the embryo. The somites, though not strictly “external features,” can be seen on external inspection, especially in the amniotes. In still later stages, the development of the appendages presents easily distinguishable and convenient characters for the definition of normal stages.
Although morphological characters appear to be the best criteria for establishing the stage of development of an embryo, there are certain limitations even to this approach. It has been found that the development of different parts (organs) of the embryo is not always strictly coordinated in time; sometimes certain ones develop more rapidly, sometimes others.
So if two embryos have certain organs (e. g., the forelimbs) in exactly the same condition, they may at the same time differ in the degree of development of other organs (e.g., the nervous system or the liver). This phenomenon of heterochrony, or unequal rate of development of parts, must always be kept in mind when any tables of normal stages are being referred to.