In this article we will discuss about the development of general body form of vertebrates.
While the various organs are being formed, the shape of the embryo as a whole undergoes far-reaching changes.
During the period of organogenesis, in the case of the vertebrate embryo, the main changes are:
i. Elongation of the body.
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ii. Formation of the tail.
iii. Subdivision of the body into head and trunk.
iv. Development of appendages.
v. Separation of the embryo proper from the extra embryonic parts.
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Some of the processes just enumerated also occur in invertebrates; in particular, the elongation of the body occurs in annelids and arthropods. The sub-division of the body into sections, such as the head, thorax, and abdomen, is typical of insects and, with modifications, of some other arthropods.
The development of appendages is an essential feature of arthropod development. Other processes concerning the body as a whole may occur in invertebrates but have no counterpart in vertebrate development. For instance, in insects the body of the embryo undergoes a peculiar shifting from the surface of the egg into the interior of the yolk, from which it emerges again at a later stage.
In holoblastic vertebrates, of which the amphibians may serve as an example, the embryo retains a spherical shape (i. e., the shape possessed by the unfertilized egg) up to the end of gastrulation and the beginning of neurulation. In the neurula stage the embryo becomes slightly elongated in an anteroposterior direction, but only after the completion of neurulation does the elongation of the embryo become prominent.
A tail rudiment, the tail-bud, appears at the posterior end of the body and rapidly develops into an elongated appendage, but the rest of the body also stretches, becoming at the same time flattened laterally and, to a certain extent, lower in a dorsoventral direction.
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Although most of the organ rudiments of the embryo are involved in this elongation, there is experimental evidence that not all are equally active. If the notochordal rudiment of an amphibian embryo in the neurula stage is excised, the embryo remains stunted and does not stretch as usual.
On the other hand, the notochordal rudiment will stretch and form an elongated rod even if it is cultivated in vitro and is not accompanied by other parts. Isolated parts of the neural system or induced brain vesicles, when no other tissues accompany them, fail to elongate.
From these facts it appears that the notochord changes its shape actively, while the nervous system is pulled in length by the adjacent notochord. In amniotes the elongation starts in the primitive streak stage, so that the body of the embryo is already long and narrow by the time the main axial organs (neural tube, notochord, dorsal mesoderm giving rise to the somites) are laid down.
In terrestrial vertebrates the sub-division of the body into the head and trunk is largely dependent on the reduction of the branchial apparatus. The system of visceral clefts and arches is fully developed in embryos of all vertebrates.
In fishes and the larvae of amphibians, the visceral clefts and arches persist and take up the area on the ventral side and posterior to the head. In terrestrial vertebrates, the branchial apparatus loses its respiratory function and becomes reduced. As a result, in later embryonic stages the area of the body posterior to the head fails to grow at the same rate as the other parts, thus producing a constricted section between the head and trunk.
The constriction is accentuated further by:
(a) A certain amount of longitudinal stretching of the neck region, as a result of which the cervical vertebrae are, as a rule, somewhat longer than thoracic vertebrae (not true in some mammals with shortened necks, such as man or the whales); and
(b) The withdrawal of the heart, originally situated in the neck region next to the branchial clefts, into the trunk (thorax).