Of the subsequent changes the two most important are (1) the formation of the gill slits or clefts; (2) the formation of the peribranchial or atrial cavity.
The formation of the gill slits is, according to Kowalevsky’s description, so peculiar that one is almost tempted to suppose that his observations were made on pathological specimens. The following is his account of the process. Shortly after the formation of the mouth there appears on the ventral line a coalescence between the epiblast and hypoblast. Here an opening is formed, and a visceral cleft is thus established, which passes to the left side, viz. the side opposite the mouth. A second and apparently a third slit are formed in the same way. The stages immediately following were not observed, but in the next stage twelve slits were present, no longer however on the left side, but in the median ventral line. There now appears on the side opposite the mouth, and the same therefore as that originally occupied by the first three clefts, a series of fresh clefts, which in their growth push the original clefts over to the same side as the mouth. Each of the fresh clefts becomes divided into two, which form the permanent clefts of their side.
The gill slits at first open freely to the exterior, but during their formation two lateral folds of the body wall, containing a prolongation of the body cavity, make their appearance ([fig. 4] A), and grow downwards over the gill clefts, and finally meet and coalesce along the ventral line, leaving a widish cavity between themselves and the body wall. Into this cavity, which is lined by epiblast, the gill clefts open ([fig. 4] B, br.c). This cavity—which forms a true peribranchial cavity—is completely closed in front, but owing to the folds not uniting completely behind it remains in communication with the exterior by an opening known as the atrial or abdominal pore.
The vascular system of Amphioxus appears at about the same time as the first visceral clefts.
Bibliography.
(1) A. Kowalevsky. “Entwicklungsgeschichte des Amphioxus lanceolatus.” Mém. Acad. Impér. des Sciences de St Pétersbourg, Series VII. Tom. XI. 1867.
(2) A. Kowalevsky. “Weitere Studien über die Entwicklungsgeschichte des Amphioxus lanceolatus.” Archiv f. mikr. Anat., Vol. XIII. 1877.
(3) Leuckart u. Pagenstecher. “Untersuchungen über niedere Seethiere.” Müller’s Archiv, 1858.
(4) Max Schultze. “Beobachtung junger Exemplare von Amphioxus.” Zeit. f. wiss. Zool., Bd. III. 1851.
(5) A. M. Marshall. “On the mode of Oviposition of Amphioxus.” Jour. of Anat. and Phys., Vol. X. 1876.
[1] The term Vertebrata is often used to include the Cephalochorda. It is in many ways convenient to restrict its use to the forms which have at any rate some indications of vertebræ; a restriction which has the further convenience of restoring to the term its original limitations. In the first volume of this work the term Craniata was used for the forms which I now propose to call Vertebrata.
[2] The protovertebræ of most embryologists will be spoken of as mesoblastic somites.
[3] The details of this process are spoken of below.
[4] The lateral position of the mouth in the embryo Amphioxus has been regarded as proving that the mouth represents a branchial cleft, but the general asymmetry of the organs is such that no great stress can, I think, be laid on the position of the mouth.