2. Increase in volume of the ovum is directly related to increase of the cytoplasm and its product, the yolk-mass. With increasing egg-size and amount of yolk, there has arisen a steadily increasing tendency to have direct development.

3. The increase of yolk has not greatly modified the process of cleavage, as segmentation in all forms is total. There is, however, a tendency to form micromeres and macromeres with increasing yolk-mass.

4. With increasing yolk-mass the wall of the blastula becomes steadily thicker and the blastocoel becomes reduced to a vestige in the animal hemisphere. The mesenchyme fails to separate as such but projects in a solid mass into the blastocoel.

5. The effect on gastrulation of increasing yolk-mass has been, first, to reduce invagination to a solid inpushing of cells; and, secondly, to bring about a subsequent epibolic inwandering of micromeres to contribute to the mesendoderm. The archenteron becomes vestigial and the definitive enteron is later excavated in the solid endoderm by a process of splitting.

6. A succession of stages in the reduction of the Ophiopluteus makes it probable that by a recession of the metamorphosis towards the gastrula stage, the larval period has been shortened, and finally lost altogether.

7. The recession of metamorphosis has resulted in the extreme case in radial symmetry being adopted immediately after the completion of gastrulation. This occurs in Kirk's ophiuroid.

8. A collateral and independent evolution has been followed by certain echinoderms with yolky eggs, resulting in the formation by convergent evolution, of a special yolk-larva, termed in this paper the “Vitellaria”. This process has taken place in the Holothuroidea, Crinoidea and Ophiuroidea, independently in each case. The Vitellaria is characterised by its cylindrical form, opacity due to yolk material present in the tissues, the complete absence of larval appendages, and the development of transverse rings of cilia.

9. With increasing yolk-mass there has occurred a reduction and loss of the enterocoels, with a corresponding increase in the amount of the mesenchyme: in this latter tissue the coelomic cavities arise by schizocoelous intercellular splitting. The hydrocoel is the last enterocoel to remain as such.

10. Viviparity appears to have been only a secondary factor in producing direct development. It has chiefly acted through prolonging embryonic development by enabling the embryo to obtain nourishment from the parent.

11. The suggestion is made that the mechanism of direct development has been an inhibitory influence upon the axial metabolic gradients of the larva. The inhibitory influence must be closely related to the presence of a yolk-mass and it has manifested itself through a steady recession of the metamorphosis toward the gastrula stage.