All the Ctenophoræ are reproduced from eggs, these eggs being so transparent that one may follow with comparative ease the changes undergone by the young while still within the egg envelope. Unfortunately, however, they are so delicate that it is impossible to keep them alive for any length of time, even by supplying them constantly with fresh sea-water, and keeping them continually in motion, both of which are essential conditions to their existence. It is therefore only from eggs accidentally fished up at different stages of growth that we may hope to ascertain any facts respecting the sequence of their development. When hatched, the little Ctenophore is already quite advanced. It is small when compared with the size of the egg envelope, and long before it is set free, it swims about with great velocity within the walls of its diminutive prison (Fig. 35). The importance of studying the young stages of animals can hardly find a better illustration than among the Ctenophoræ. Before their extraordinary embryonic changes were understood, many of the younger forms had found their way into our scientific annals as distinct animals, and our nomenclature thus became burdened with long lists of names which will disappear as our knowledge advances. The great size of their locomotive flappers in proportion to the rest of the body, is characteristic of the young Ctenophoræ. They seem like large paddles on the sides of these tiny transparent spheres, and, owing to their great power as compared with those of the adult, the young move with extraordinary rapidity. The Pleurobrachia alone retains its quickness of motion in after life, and although its long graceful streamers appear only as short stumpy tentacles in the young (Fig. 34), yet its active little body would be more easily recognized in the earlier stages of growth than that of the other Ctenophoræ. Figs. 34, 35, and 36 show the Pleurobrachia at various stages of growth; Fig. 34, with its thick stunted tentacles and short rows of flappers, is the youngest; the flappers themselves are rather long at this age, looking more like stiff hairs than like the minute fringes of the adult. In Fig. 35 the tentacles are already considerably longer and more delicate; in Fig. 36 the vertical tubes are already completed, while Figs. 27-29 present it in its adult condition. [Illustration: Fig. 34. Young Pleurobrachia still in the egg; _t_ tentacle, _e_ eye-speck, _c_ _c_ rows of locomotive flappers, _d_ digestive cavity; greatly magnified.] [Illustration: Fig. 35. Young Pleurobrachia swimming about in the egg just before hatching; magnified.] [Illustration: Fig. 36. Young Pleurobrachia resembling somewhat the adult; _f_ funnel leading to anal opening, _l_ lateral tubes, _o_ _o_ _o'_ _o'_ rows of locomotive flappers; magnified.] [Illustration: Fig. 37. Young Idyia, greatly magnified; lettering as in Fig. 36; _d_ digestive cavity.]