ii. ii LARVA OF AMPHIOXUS 45 



stage' with eight pairs of slits the larva pauses for some time before 

 further changes. It is interesting that this is the time at which it most 

 nearly represents what might have been an ancestral craniate, with 

 eight branchial arches (p. 145). Further slits are then gradually added 

 in pairs on both sides. Each slit becomes subdivided, soon after its 

 formation, by the downgrowth of a tongue bar. The atrium is absent 

 from the early larva. Metapleural folds then appear on either side and 

 are united from behind forwards to form a tube below the pharynx. 

 During the later stages of development the larva sinks and finally rests 

 on the bottom while undergoing the migration of gill-slits that con- 

 stitutes its metamorphosis. In other species the larva remains longer 

 in the plankton, becoming large and even showing quite large gonad 

 rudiments. These were at first thought to be adults of a new genus 

 (Amphioxides). 



The development of amphioxus, like its adult organization, shows 

 us many features of the plan that is typical of all chordates and was 

 presumably present in the earliest of them. Thus the cleavage, in- 

 vagination, and mesoderm formation recall those of echinoderms and 

 other forms similar to the ancestors of the chordates, and also show a 

 pattern from which all later chordate development can be derived. 

 Unfortunately we cannot pursue this study as far as we should like 

 because of the difficulty of investigating the development of am- 

 phioxus. Modern embryologists aim at tracing the morphogenetic 

 movements by which the organism is built, and ultimately at dis- 

 covering the forces responsible for these processes. We still remain 

 ignorant of the details of these morphogenetic movements, and can 

 only guess that the system of cell activities by which an amphioxus 

 is built represents quite closely the original set of morphogenetic 

 processes of vertebrates (Young, 1957, p. 633). 



There are, of course, some special features connected with the 

 method of life of the larva, and especially with its asymmetry. The 

 strange sequence of gill formation, the immense left-sided larval 

 mouth, perhaps the club-shaped gland, and Miiller's organ, may show 

 considerable modifications of relatively recent date. However, the 

 earliest chordates probably fed by means of cilia and were planktonic, 

 so we must not too hastily assume that even these asymmetrical features 

 are novelties. 



The division of the mesoderm of amphioxus into a series of sacs 

 presents an interesting problem. The segmentation of the mesoderm 

 of vertebrates is restricted to the dorsal region. In the lowest chord- 

 ates (see p. 51), as in their pre-chordate ancestors, there are three 



