BY T. HARVEY JOHNSTON AND O. \V. TIEGS. 221 



whilst from the peritoneum lining them the ova are developed. 

 The organ is slightly elongate transversely, and lies ventrally 

 and laterally from the rectum. It thus has a position different 

 from that described for Bonellia, where its main axis is longi- 

 tudinal. 



Spengel (1897, pp. 360-373) gave a detailed account of the 

 early development of the ova of B. viridis. We were not able 

 to observe the early stages, while the later ones are different 

 from those described by him. He gave an account of character- 

 istic cell masses developing from the peritoneum lining the 

 ventral bloodvessel, and forming each a spherical mass consisting 

 of a relatively large central cell (which degenerates later) sur- 

 rounded by a layer of small peripheral cells. It is from one of 

 the latter that, according to him, the ovum develops at the 

 expense of the remainder. 



In our specimens the earliest stage observable shows this cell 

 mass in its nine-celled condition, there being a slightly differ- 

 entiated central cell present (Plate x., fig. 16a). It is this latter 

 cell that develops into the ovum. The cell mass grows quickly, 

 the central cell outstripping the others, which become cubical 

 and form a membrane, at first thick and closely fitting, but later 

 thin and far separated from the central cell (Plate x., figs.l66-e). 

 In these later stages, the central egg-cell can be seen to be actu- 

 ally connected by a stalk to the mesenteric strands. As the egg 

 grows larger, it fills the space within the ring of accessory cells; 

 and, continuing to grow, stretches this layer, which eventually 

 comes to lie as a delicate membrane around the egg. The latter 

 is now fully developed. At this stage the stalk evidently rup- 

 tures and the egg escapes. Within the coelome, eggs may be seen 

 with the small peripheral cell nuclei on their surface. 



The ova pass forward and are received into the uteri. The 

 peripheral egg membrane has by this time disappeared. In 

 Bonellia, according to Spengel, it bursts before reaching the 

 uterus. The eggs in the uteri are large cells, each with a pro- 

 minent nucleus. They measure about OTlmm. in diameter, 

 while the nuclei are about 0'037 mm. across. The latter contain 

 large numbers of nucleoli. The egg cytoplasm is faintly granular, 



