of an Asterid icith. Large Yolkij E'jgs. 391 



Immediately beneath the bases of the ectoderm cells, 

 where these abut on the perihsemal sjjaces, it is possible to 

 trace evidence of what I conclude to be the nervous system. 

 This appears as a layer of exceedingly fine fibrils, which 

 could only be made out by a high power, and which 

 apparently replaces the mesenchyme intervening between the 

 ectoderm and the walls of the perihsemal spaces (PI. XIII. 

 fig. I). 



The cells of the peritoneum vary in shape from flattened 

 to cubical, and occasionally one sees evidence of fine myo- 

 epithelial cells, the beginning of muscle-formation. The 

 ectoderm, too, is often bent into folds, while the peritoneum 

 remains straight, a fact which may be regarded as evidence 

 of the contractile power of the latter. This is especially true 

 of the preoral lobe, the ectoderm of which becomes folded 

 into pockets, while the peritoneum of the coelom assumes 

 a contracted appearance. These folds are closed off from 

 the exterioi- and are then destroyed by amoebocytes. 



The cells of the hydrocoele vary in appearance from 

 columnar to flattened or cubical. In the radial water 

 vascular canal they are flat or cubical, while in the tube-feet 

 and terminal tentacles they are columnar and increased in 

 number, so that the nuclei appear several layers deep (PL XIII. 

 fig. I). Fig. I also shows muscular elements developing 

 on the aboral side of the walls of the perihsemal spaces 

 {muse). These muscles would serve, when in action, to 

 draw together the ambulacral ossicles of opposite sides and 

 so close the ambulacral groove in the adult. 



PI. XIII. Hg. J shows a curious group of cells projecting 

 into the axial sinus from its posterior wall. This is the ovoid 

 gland, and is composed of a mass of primitive germ-cells. 

 These are large and cylindrical in shape, held together by 

 fibres derived from the mesenchyme. In places the surface of 

 this group of cells is irregular and folded, and covered with 

 smaller cells, which are doubtless derived from the peri- 

 toneum of the axial sinus, Avhich has been pushed in before 

 it in its growth forward. Further, it is developed from the 

 wall of the left posterior coelom, wliich becomes thickened 

 and invaginated. 



The foregoing are the chief points which could be made 

 out from the material. I have endeavoured to show, in my 

 thesis, of which this is an abstract, the close parallelism in 

 development between this organism and Asterina; while here 

 I have mentioned them but briefly, and rather dwelt on the 

 more important diflerences, and some points of technique, 

 which I have learned while doing the laboratory work. 



