The Development of Phascolosoma. 107 



A narrow longitudinal band of ectoderm, which I have called 

 (1903, 1904) the dorsal cord, extends through this partial inter- 

 ruption of the prototroch (Fig. 47, 89 cd. d). In its narrow middle 

 part, where it is overlapped by the prototroch, it consists a double 

 or single row of cells, thus resembling the corresponding cord which 

 underlies the dorsal amniotic canal in Sipunculus, It rapidly widens 

 in front, and merges into the apical plate. It is continuous behind 

 (Fig. 89) with the dorsal ectoderm of the trunk. The prototroch 

 cells are covered uniformly with short adorai cilia (Fig. 49, 82). 



A narrow band of cells occurs immediately behind the prototroch, 

 which probably gives rise to the larval mesenchyme, or primary 

 mesoderm cells, which become converted in the older trochophore 

 into the prominent circular muscles of this region (Fig. 85) and 

 into the accessory retractors (Fig. 88, 96). Directly behind this 

 band in Ph. vidgare is a complete ring of relatively large cells, 

 which bear the postoral circlet of cilia (Fig. 85). A similar row of 

 somewhat prominent cells is visible in sections of Ph. goiddii (Fig. 87), 

 but in this species cilia are scantily represented upon them, and 

 the circlet may be regarded as a vestigial structure. There is no 

 paratroch in Phascolosoma. 



The beginnings of the supraoesophageal ganglion were described 

 in connection with the apical plate from which it arises. The rudi- 

 ment of the nerve cord arises quite independently of the ganglion, 

 as an unpaired thickening of ectoderm along the median ventral line. 

 The differentiation takes place first along the lateral margin, as in 

 Sipunciilus. It is an important fact in the consideration of the 

 relationships of the Sipunculids that there is no concrescence of 

 lateral rudiments to form the nerve cord as in Chaetopods, for it 

 is fundamentally a single unpaired band. As soon as the nerve 

 cord in Ph. gouldii becomes cut off in front from the overlying ecto- 

 derm, it is found to be divided into from two to four distinct 

 segments (Fig. 86, 89). 



A pair of pole cells of the coeloraesoblast can be distinguished 

 at twenty four hours, immediately behind the stomodaeal invagination. 

 From them the bands extend dorsad and forward (Fig. 43, 45). The 

 mesoderm of the trochophore of thirty-six hours (Fig. 48) completely 

 fills the space in the posterior hemisphere behind the archenteron, 

 and extends forward dorso-laterally beneath the prototroch cells, 

 until it finally touches the dorsal side of the apical plate. 



Distinct signs of segmentation of the mesoblast are visible in 



