A laboratory text-book of embryology / by Charles Sedgwick Minot.

  • Charles Sedgwick Minot
Date:
1911
Catalogue details

Licence: Public Domain Mark

Credit: A laboratory text-book of embryology / by Charles Sedgwick Minot. Source: Wellcome Collection.

    18/428 page 2
    The Segmented Animals. Metamerism. All vertebrates and certain invertebrates have their bodies divided into a series of parts, which begins in the region of the head and extends to the caudal end of the body. These parts are called segments or metameres. In man each vertebra corresponds to one segment. In the earthworm and other annelids the segments are plainly marked on the outer surface of the body. Since all segmented animals are bilaterally symmetrical, each segment is bilaterally sym¬ metrical and may therefore be described as a paired structure. Embryology has demonstrated that each segment arises as a pair of masses (somites) situated between the digestive canal and the outer surface of the body. Each pair of masses is formed from the middle germ-layer (mesoderm) exclusively (compare pages 84-85) and is known as “a primitive segment.” The mesodermic somites are to be considered the essential primary morphological segments, and in the course of the development of the individual they produce adult metameric structures, among which may b£ muscles, skeletal elements, excretory organs, etc., the whole history of which depends upon their segmental origin. The nervous system and to a considerable extent the blood-vessels exhibit a segmental arrangement, which is usually regarded as a secondary correlation of these structures with the primary mesodermic meta'merism. The spinal nerves exemplify the correlation. In brief, where the segmental organization exists, it dominates the anatomy alike of the embryo and the adult; therefore the student of embryology should pay special attention to the segmentation of the body in all its chief stages. In regard to the bodily segmentation two general observations may be made: First, the development of segments begins at the cephalic end and progresses tailward; hence so long as the development of segments continues various stages of their differentiation may be found in a single embryo, the more advanced stages being always cephalad from the less advanced. Second, there is a funda¬ mental difference between the metamerism of vertebrates and that of annelids and many other invertebrates, which consists in the unlike extent of the segments; for the primitive segments of vertebrates are confined to the dorsal region of the body, while in the other forms the segmentation extends from the start through the dorsal and ventral regions both. It is probable that the segments of vertebrates are homologous only with the dorsal part of the segments of annelids. The Vertebrate Type of Structure. When one traces the course of development of any vertebrate, one finds, speaking in general terms, that the fundamental characteristics, which are more or less common to all vertebrates, are those which first appear. Later, there come in the secondary characteristics which distinguish one class from another, and still later the subordinate characteristics by which the smaller subdivisions of the vertebrate type become differentiated one from another. This statement, however, is correct only if we add to it certain indispensable limitations. Every embryo at every stage of its development is an individual of the particular genus
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