Form and Function: A Contribution to the History of Animal Morphology

by E. S. Russell

The next big step in the study of the development of the skull was taken by a pupil of Müller, C. B. Reichert, who showed in his work very distinct traces of his master's influence. Reichert's first and most important contribution to the subject was his paper on the metamorphosis of the gill, or, as he called them, the visceral arches in Vertebrates,[204] particularly in the two higher classes. Reichert describes the similar origin in embryo of bird and mammal (pig) of three "visceral" arches. These arches stand in close relation to the three cranial vertebræ which Reichert, like Müller, distinguishes. He makes the retrograde step of admitting only three aortic arches, and he is not inclined to consider the three visceral arches as equivalent to the gill-arches of fish—in his opinion they have more analogy with ribs, though differing somewhat from ribs in their later modifications. The visceral arches are processes of the visceral plates (von Baer), which grow downwards and meet in the middle line, leaving between one another and the undivided body wall three visceral slits opening into the pharynx. The first visceral process is different in shape from the others, for it sends forward, parallel with the head and at right angles to its downward portion, an upper portion in which later the upper jaw is formed. The other two processes are straight. From the hinder edge of the second visceral arch there develops, as Rathke had seen, a fold which is comparable with the operculum of fish. The first slit develops externally into the ear-passage, internally into the Eustachian tube, and in the middle a partition forms the tympanic ring and tympanum. Inside each of the visceral processes on either side a cartilaginous rod develops. In the first process this rod shows three segments, of which the first lies inside that portion of the process which is parallel with the head. This upper segment forms the foundation for the bones of the upper jaw. The lowest segment of the cartilaginous rod becomes Meckel's cartilage, and on the outer side of this the bones of the lower jaw are formed. The middle segment becomes in mammals the incus (one of the ear-ossicles), and in birds the quadrate. Meckel's cartilage, which was discovered by Meckel[205] in fish, amphibians and birds, is a long strip of cartilage which runs from the ear-ossicle known as the hammer in mammals,[206] to the inside of the mandible. Reichert shows how this relation comes about.

Fig. 9.—Meckel's Cartilage and Ear-ossicles in Embryo of Pig. (After Reichert.)

a. Mandible.	h. Hammer.	k. Incus.
g. Meckel's cartilage.	i. Handle of Hammer.	n. Stapes.

The hammer, according to his observations on the embryo of the pig, is simply the proximal end of Meckel's cartilage, which later becomes separated off from the long distal portion (see Fig. 9). The third ear-ossicle of mammals, the stapes, comes not from the first arch but from the second. The cartilaginous rod of the second arch segments like the first into three pieces. Of these the uppermost disappears, the middle one, which lies close up to the labyrinth of the ear, becomes the stapes, and the lowest becomes the anterior horn of the hyoid. The stapes forms a close connection with the hammer and the incus. In birds, where there is a single ear-ossicle, the columella, the middle piece of arch I forms, as we have seen, the quadrate, by means of which the lower jaw is joined to the skull. The proximal end of Meckel's cartilage, which in mammals forms the hammer, here gives the articular surface between the lower jaw and the quadrate. The columella is formed from the middle piece of the three into which the cartilage of the second arch segments. It is, therefore, the homologue of the stapes in mammals. The third arch takes a varying share, together with the second, in the formation of the hyoid apparatus.

In this paper Reichert made a distinct advance on the previous workers in the same field—Rathke, Huschke, von Baer, Martin St Ange, Dugès. Huschke was indeed the first to suggest that both upper and lower jaws were formed in the first gill-arch. But both von Baer and Rathke[207] held that the upper jaw developed as a special process independent of the lower jaw rudiment, and the actual proof that the upper jaw is a derivative of the first visceral arch seems to have been first supplied by Reichert. His brilliant work on the development of the ear-ossicles founded what we may justly call the classical theory of their homologies. His views were attacked and in some points rectified, but the main homologies he established are even now accepted by many, perhaps the majority of morphologists.

In a paper of 1838 on the comparative embryology of the skull in Amphibia,[208] Reichert added to his results for mammals and birds an account of the fate of the first and second visceral arches in Anura and Urodela.

The first visceral arch, he found, gave in Amphibia practically the same structures as in the higher Vertebrates. Its skeleton segmented, as in mammals and birds, into three parts; the upper part gave rise to the palatine and pterygoid in Anura, but seemed to disappear in Urodeles, where the so-called palatine and pterygoid developed in the mucous membrane of the mouth; the middle part gave, as in birds, the quadrate, which formed a suspensorium for both arches; the lower part, as Meckel's cartilage, formed a foundation for the bones of the lower jaw. Of arch II., the lower part became the horn of the hyoid, the upper part had a varying fate. In some Anura it formed the ossicle of the ear (homologue of the columella of birds and the stapes of mammals), in others it disappeared. In reptiles the upper segment of the second arch formed, as in birds, the columella.

The account of the metamorphoses of the visceral arches in Amphibia forms only a small part of Reichert's memoir of 1838, the chief object of which was to discover the general "typus" of the vertebrate skull, and to follow out its modifications in the different classes. Von Baer had shown that the generalised type appeared most clearly in the early embryo; Reichert therefore sought the archetype of the skull in the developing embryo. He brought to his task the preconceived notion that the skull could be reduced to an assemblage of vertebræ, but he saw that comparative anatomy alone could not effect this reduction; he had recourse, therefore, to embryology, hoping to find in the simplified structure of the embryo clear indications of three primitive cranial vertebræ (p. 121, 1837).