Bonellia. The embryo of Bonellia, while still within the egg, retains a spherical form and acquires an equatorial band of cilia, behind which a second narrower band is soon established, while in front of the first one a pair of eye-spots becomes formed (fig. 162 A). The embryo on becoming hatched rapidly elongates, while at the same time it becomes dorso-ventrally flattened and acquires a complete coating of cilia (fig. 162 B). According to Spengel it resembles at this time in its form and habits a rhabdocoelous Turbellarian. The anterior part is however somewhat swollen and presents an indication of a præ-oral lobe. [FIG. 162. THREE STAGES IN THE DEVELOPMENT OF BONELLIA. (After Spengel.) A. Larva with two ciliated bands and two eye-spots. B. Ripe larva from the dorsal surface. C. Young female Bonellia from the side. _al._ alimentary tract; _m._ mouth; _se._ provisional excretory tube; _s._ ventral hook; _an.v._ anal vesicle.] During the above changes important advances are made in the formation of the organs from the embryonic layers. The epiblast acquires a superficial cuticula, which is perhaps directly derived from the vitelline membrane. The nervous system is also formed, probably from the epiblast. The band-like supra-oesophageal ganglion is the first part of the nervous system formed, and appears to be undoubtedly derived from the epiblast. The ventral cord arises somewhat later, but the first stages in its development have not been satisfactorily traced. It is continuous with the supra-oesophageal band which completely girths the oesophagus without exhibiting any special dorsal enlargement. After the ventral cord has become completely separated from the epiblast a central fibrous mass becomes differentiated in it, while the lateral parts are composed of ganglion cells. In the arrangement of its cells it presents indications of being composed of two lateral halves. It is, however, without ganglionic swellings. The mesoblast, though at first very thin, soon exhibits a differentiation into a splanchnic and somatic layer--though the two do not become distinctly separated by a body cavity. The somatic layer rapidly becomes thicker, and enlarges laterally to form two bands united dorsally and ventrally by narrow, thinner bands. The outermost parts of each of these bands become differentiated into an external circular and an internal longitudinal layer of muscles. In the præ-oral lobe the mesoblast assumes a somewhat vacuolated character. The hypoblast cells form a complete layer round the four yolk cells from which they arise (fig. 162 B, _al_), but at first no alimentary lumen is developed. The oesophagus appears during this period as an, at first solid, but subsequently hollow, outgrowth of the hypoblast towards the epiblast.