1. The first cleavage may be either equal, or markedly unequal; when it is equal the next segmentation affects both blastomeres; when it is unequal the larger blastomere is believed to give rise to three cells, and the smaller remains undivided till the next cleavage.
2. At the eight-cell stage there are two quartets of blastomeres. The upper quartet, micromeres, occupy the animal pole. The lower quartet, macromeres, occupy the vegetal pole.
3. The blastula comprises micromeres and macromeres, and the blastocoel is small and becomes eccentric. No cilia are developed.
4. The gastrula is formed by the shallow imagination of the macromeres, accompanied by an extensive process of epiboly affecting the micromeres. More marked epiboly of cells on two sides of the blastomere produces in the early stages two crests which later disappear. These may indicate a trace of bilateral symmetry. Epiblast comes to lie on solid mes-hypoblast. The archenteron is transient, and gives rise to no structures. The blastopore occupies the position of the definitive mouth.
5. No larva ever forms, nor is there any vestige of a larval stage.
6. The solid gastrula is converted into the adult by assuming a radial symmetry directly, with no intermediate bilaterally symmetrical form, unless the two epibolic crests are regarded as vestiges of larval symmetry.
7. The podia appear as solid outgrowths, in which the hydrocoelic cavity develops by splitting.
8. The definitive enteron appears as a split extending upward from the ventral surface through the solid hypoblast.
9. The young ophiuroid leaves the egg before the appearance of the general body cavity, and moves about, but does not at first take food.
10. The general coelomic body cavity and the perihaemal cavity develop by splitting in a mass of mesenchyme derived from the outer layers of mes-hypoblast.
11. The formation of the skeletal system is delayed till the stage of between two and three arm-segments.
12. The development of the skeleton follows closely that described for Amphiura squamata.
13. The tooth is shown to originate independently of the torus angularis; its rudiments comprise nine symmetrically disposed spicules.
14. The terminal plate arises later than the radials, and has a distinctive ‘primitive structure’.
15. The spine is shown to have a different development to that of the tooth, and therefore would seem to have no connexion with the latter in phylogeny or ontogeny.
16. It is suggested that the aberrant early stages are to be correlated with the retarding effect of the yolk mass present in the egg during ontogeny. The aberrant features may have had a different origin in phylogeny.
17. It is suggested that the simultaneous appearance in ontogeny of homologous organs situated at equal radial distances from the centre is to be explained in terms of hormonic activity.
18. It is concluded that evolution has considerably affected the early ontogeny without leaving its mark on phylogeny. The adult thus conforms to its class, the young form does not.
Establishment of the axis in chordates: facts and speculations