The Origin of Vertebrates

by Walter Holbrook Gaskell

The evidence all points directly to the second method of formation. Thus Shipley, in his description of the development of the lamprey, says—

"The gill-slits appear to me to be the result of the ventral downgrowth of mesoblast taking place only at certain places, these forming the gill-bars. Between each downgrowth the hypoblastic lining of the alimentary canal remains in contact with the epiblast; here the gill-opening subsequently appears about the twenty-second day."

Dohrn describes and gives excellent pictures of the growth of the diaphragms, as the Ammocœtes grows in size, pictures which are distinctly reminiscent of the corresponding illustrations given by Brauer of the growth of the internal gills in the scorpion embryo.

Another piece of evidence confirmatory of the view that the branchial segments are really of the nature of internal appendages, as the result of which gill-pouches are formed, is given by the presence in each of these branchial bars or diaphragms of a separate cœlomic cavity. From the walls of this cavity the branchial muscles and cartilaginous bar are formed.

Now, from an embryological point of view, the vertebrate shows that it is a segmented animal by the formation of somites, which consist of a series of divisions of the cœlom, of which the walls form a series of muscular and skeletal segments. In the head-region, as already mentioned, such cœlomic divisions form two rows—a dorsal and a ventral set. From the walls of the dorsal set the somatic musculature is formed. From those of the ventral set the branchial musculature. From the latter also the branchial cartilaginous bars are formed. Thus Shipley, in his description of the development of the lamprey, says: "The mesoblast between the gills arranges itself into head-cavities, and the walls of these cavities ultimately form the skeleton of the gill-arches."

Similarly, in the arthropod, the segments in the embryo are marked out by a series of cœlomic cavities and Kishinouye has described in Limulus a separate cœlomic cavity for every one of the mesosomatic or branchial segments, and he states that in Arachnida the segmental cœlomic cavities extend into the limbs. These cavities both in the vertebrate and in the arthropod disappear before the adult condition is reached.

The whole evidence thus points strongly to the conclusion that the true branchial segmental units are the branchial bars or diaphragms, not the pouches between them.

It is possible to understand why such prominence has been given to the conception of the branchial unit as a gill-pouch rather than as a gill-appendage, when the extraordinary change of appearance in the respiratory chamber of the lamprey which occurs at transformation, is taken into consideration. This change is of a very far-reaching character, and consists essentially of the formation of a new alimentary canal in this region, whereby the pharyngeal chamber of Ammocœtes is cut off posteriorly from the alimentary canal, and is confined entirely to respiratory purposes, its original lumen now forming a tube called the bronchus, which opens into the mouth and into a series of branchial pouches.

In Fig. [68] I give diagrammatic illustrations taken from Nestler's paper to show the striking change which takes place at transformation, (A) representing three branchial segments of Ammocœtes, and (B) the corresponding three segments of Petromyzon. The corresponding parts in the two diagrams are shown by the cartilages (br. cart.), the sense-organs (S), and the branchial veins (V. br.); the corresponding diaphragms are marked by the figures 1, 2, 3 respectively. As is clearly seen, it is perfectly possible in the latter case to describe the respiratory chamber, as Nestler has done, as divided into a series of separate smaller chambers—the gill-pouches—by means of a series of diaphragms or branchial bars. The surface of these gill-pouches is in part thrown into folds for respiratory purposes, and each gill-pouch opens, on the one hand, into the bronchus (Bro.), and, on the other, to the exterior by means of the gill-slit. The branchial unit in Petromyzon is, therefore, according to Nestler and other morphologists, the folded opposed surfaces of two contiguous diaphragms, and each one of the diaphragms is intersegmental between two gill-pouches.

Nestler then goes on to describe the arrangement in Ammocœtes in the same terms, although there is no bronchus or gill-pouch, but only an open chamber into which these gill-bearing diaphragms project, which open chamber serves both for the passage of food and of the water for respiration. This is manifestly the wrong way to look at the matter: the adult form is derived from the larval, not vice versâ, and the transformation process shows exactly how the gills, in Rathke's sense, come together to form the bronchus and so make the gill-pouches of Petromyzon.