THE DEVELOPMENT OF OPHTOTHRIX FRAGILIS. 571 



stages differ considerably from one another. From eggs 

 artificially fertilised larv£C are obtained which are shown in 

 Pis. 32 and 33, figs. 18-23, whilst when a male and female 

 are brought together and allowed to spawn naturally the 

 fertilised eggs develop into larvse, the corresponding stages 

 of which are shown in PI. 31, figs. 1-5, and PI, 33, figs. 24-28. 

 Considering first the normal development due to natural 

 fertilisation we find that the eggs, which are small (about 

 "1 mm. in diameter) and opaque, owing to the presence of 

 a yellow yolk, develop in seven or eight hours into small thick- 

 walled blastulse (PI. 31, figs. 1 and 2), from one side of which 

 mesenchyme cells are budded off (PI. 33, fig. 24, mes.). At 

 the end of twelve to eighteen hours the embryo has escaped 

 from the egg membrane and become ciliated and free-swim- 

 ming, and is now henceforth to be designated a larva. The 

 larva is egg-shaped (PI. 31, fig. 3), the more pointed end, which 

 is directed forwards, being formed of a great crest of vacuolated 

 cells, which possibly acts as a hydrostatic apparatus (PI. 33, 

 fig. 25), whilst at the opposite pole the formation of mesen- 

 chyme is going on, and the invagination which is to form 

 the primary gut or archenteron is beginning (PI. 33, 

 figs. 25-27). During the next fifteen or twenty hours the 

 form of the larva changes Jrom an egg-shape to a lozenge- 

 shape, owing to the appearance of two lateral outgrowths, 

 which are the earliest rudiments of the postero-lateral arms 

 of the larva (PL 33, fig. 28, p/.). Into these outgrowths 

 the most of the mesenchyme cells wander, where they form 

 the skeletogenous basis of the calcareous rods which support 

 these arms. The space between the archenteron and the 

 ectoderm is of course identical with the blastoccele or cavity of 

 the blastula; it has been termed the archiccele or primary 

 body-cavity by German authors; both these names will be 

 employed in this paper. The secondary body-cavity or ccelom 

 (PI. 33, figs. 28 and 29, ae.), arises as a thin-walled bilobed 

 outgrowth from the apex of the archenteron when the larva 

 is from thirty to forty hours old. The postero-lateral arms 

 grow rapidly and the vacuolated crest dwindles in similar 



