1. The breeding season of Cucumaria echinata seems to begin in the middle of June and to last until the oarly part of August. During that season the wall of the genital tubes is thin, but in an inactive period it is very thick. No muscle layer could be made out in the wall. The genital papilla is subdivided, the branches being more numerous in males than in females. Both sexes occur in almost equal numbers. 2. The ovarian egg is attached to the wall of the genital tube by its broad vegetative half. At the animal pole which is directed towards the internal lumen of the tube a short rod-like cytoplasmic process is found. This structure develops near the end of the growth of the egg, and probably has some significance in relation to future changes of the egg. 3. Freshly captured mature animals spawn in the evening. At first the males shed out spermatic fluid, and after some minutes the females begin to lay eggs. During these acts no special movements of tentacles are observed in either sexes. 4. The newly-shed egg is slightly flattened and measures about 300--400 µ in diameter. It is covered with a gelatinous layer, through which a canal opens at the animal pole. The egg is heavier than sea-water. 5. The first polar body has been formed by the time it is shed, when the second maturation spindle is to be seen. The spermatozoon enters the egg before the second maturation division, and probably at the point near to, but not precisely identical with, the animal pole. 6. The first cleavage spindle is formed within an hour. The cleavage is total and equal, proceeding quite regularly up to about the thirty-two-cell stage. Very often an interlocking of blastomeres occurs. Inequality in size of the blastomeres is met with above the thirty-two-cell stage, and the embryo is wrapped up within the egg-membrane until the blastula stage has been attained. 7. The blastula is spherical but not wrinkled, and is now free from egg-membrane. It swims about by means of cilia. The mesenchyme-formation precedes imagination, occurring exclusively at the vegetative pole. The invagination begins the next morning. 8. In a fully-formed gastrula the archenteron shows a peculiar twisting, enabling one to distinguish in it three parts. The most anterior flat part is the future hydrocoele, the second transverse part is the, future enterocoele, and the hindermost tubular part is the future gut. 9. Very late in the gastrula stage the stomodaeum makes its first appearance, being preceded by a thickening of the ectoderm at about the middle of the ventral side. Borne mesenchyme cells seem to be formed here by the proliferation of ectodermal cells. The position of the stomodaeum is, as can be shown in later stages, a little on the left of the median line. 10. The dipleurula stage begins late on the second day. In this stage the hydro-enterocoele first becomes separated from the gut. The former then divides into the hydrocoele and enterocoele. The hydrocoele produces the rudiment of the pore-canal directed postero-dorsad, and six lobes on the anterior expanded margin. These latter are rudiments of the five primary tentacles and of the mid-ventral radial canal. The enterocoele divides into right and left vesicles, situated on the left dorsal and antero-ventral sides respectively. 11. On the third day doliolaria is formed, which is characterized by the possession of three ciliary bands around the posterior half of the body besides the weaker uniform ciliation over the pro-oral hood and on the anal field. From the hydrocoele are first differentiated the mid-ventral radial canal and four of the primary tentacles. 12. The primary pair of pedicels make their appearance as ectodermal depressions (pedal pits) situated between the second and third ciliary bands. The left pedicel is a little earlier in appearing than the right, while neither of the two can be said definitely to be anterior to the other in position. 13. The original position of the primary tentacles is decidedly interradial, but their bases gradually shift towards the respective radial canal according to a definite asymmetrical feature. The one in the left dorsal interradius appears last. 14. The Polian vesicle appears at the free end of the ventral limb of the hydrocoele ring, while about the same time the axial sinus is formed as a secondary dilatation of the middle part of the pore-canal. The dorsal pore has now opened between the second and third ciliary bands. 15. The hydrocoele ring closes in the left dorsal interradius. This is clearly shown by the position of the rudiments of the dorsal and ventral radial canals of the left side, appearing usually before the closure of the ring. Of the four paired radial canals the right dorsal appears first, while the left ventral is the last to appear. 16. Fusion of the right and left enterocoeles occurs on the right side, while on the other side the two vesicles lie close but separated. This intervening portion gives rise to the mesentery,which at last bends in an S-shape in agreement with the coil of the gut in the future. The gut is almost solid, leaving but very narrow lumen. Blastocoele jelly is most massive in the doliolaria stage, and mesenchyme cells thickly cover all the internal vesicles, without, however, forming any definite cell-layer. 17. The latter half of the doliolaria stage may be distinguished by calling it metadoliolaria. Here degeneration of the pre-oral hood and ciliary bands sets in, while muscles and nerves are differentiated, besides the further completion of hydrocoele and enterocoele. Calcareous deposits, too, make their first appearance in this stage. They appear in three places : the wall of the axial sinus, the bases of the tentacles, and the integument of the posterior part of the body. 18. In the course of a week or more the larva changes into a creeping stage, pentactula. The five tentacles have now a few branches and the third pedicel appears at last. The gut is now open throughout,...