The Origin of Vertebrates

by Walter Holbrook Gaskell

Here, in contradistinction to the cranial region, the segmentation is most marked, for the somatic spinal musculature of all vertebrates can be traced back to a simple sheet of longitudinal ventral and dorsal muscles, such as are seen in all fishes. This sheet is split into segments or myotomes by transverse connective tissue septa or myo-commata; each myotome corresponding to one spinal segment.

In addition to the evidence of segmentation afforded by the body-musculature in all the higher vertebrates, similar evidence is given by the segmental arrangement of parts of the supporting tissue to form vertebræ. Such segments have received the name of sclerotomes, and each sclerotome corresponds to one spinal segment.

Yet another marked peculiarity of this region is the segmental arrangement of the excretory organs. Just as our body-musculature has arisen from the uniformly segmented simple longitudinal musculature of the lowest fish, so, as we pass down the vertebrate phylum, we find more and more of a uniform segmental arrangement in the excretory organs.

The origin of all these three separate segmentations may, in accordance with the phraseology of the day, be included in the one term—the origin of the spinal mesoblastic segments—i.e. of the cœlomic cavities of the trunk-region and the structures derived from their walls.

The Origin of the Segmental Excretory Organs.

Of these three clues to the past history of the spinal region, the segmentation manifested by the presence of vertebræ is the least important, for in Ammocœtes there is no sign of vertebræ, and their indications only appear at transformation. Especially interesting is the segmentation due to the excretory organs, for the evidence distinctly shows that such excretory organs have steadily shifted more and more posteriorly during the evolution of the vertebrate.

In Limulus the excretory organs are in the prosomatic region—the coxal glands; these become in the vertebrate the pituitary body.

In Amphioxus the excretory organs are in the mesosomatic region, segmentally arranged with the gills.

In vertebrates the excretory organs are in the metasomatic region posterior to the gills, and are segmentally arranged in this region. Their investigation has demonstrated the existence of three distinct stages in these organs: 1. A series of segmental excretory organs in segments immediately following the branchial segments. This is the oldest of the three sets, and to these organs the name of the pronephros is given. 2. A second series which extends more posteriorly than the first, overlaps them to an extent which is not yet settled, and takes their place; to them is given the name of the mesonephros. 3. A third series continuous with the mesonephric is situated in segments still more posterior, supplants the mesonephros and forms the kidneys of all the higher vertebrates. This forms the metanephros.

These three sets of excretory organs are not exactly alike in their origin, in that the pronephric tubules are formed from a different portion of the cœlomic walls to that from which the meso- and metanephric tubules are formed, and the former alone gives origin to a duct, which forms the basis for the generative and urinary ducts, and is called the segmental duct. The mesonephric tubules, called also the Wolffian body, open into this duct.