Plate X. 



Figs. 1 — 9. Successive vertical sections, parallel to the sagittal plane, through an embryo in the stage III, 

 prepared out of the egg-membrane. ^°/j. 



Fig. I. The section goes through the outer part of the tentacle sheath. 



Fig. 2. Further inwards, the section going throu.gh the coiled-up tentacle which lies within 

 the tentacle sheath. (Conip. PI. II. Fig. 8, PI. VIII. Fig. 12). The outer end of the entodermal 

 sacs is seen in the section; here the large entodermal cells are lying in two strands, as 

 in the branching peripheral canal system. 



Figs. 3 — 7 give successive sections through the tentacle apparatus and the entodermal sacs. 

 Fig. 8 is inside the tentacle basis. In fig. 6 is seen the direct connection between the 

 lumen of the entoderm sacs and the diverticula in the tentacle basis. Figs. 7— 8 show 

 the continuation of the costal sacs as low furrows running towards the apical organ. 



Fig. 9. Median section, through the apical organ, the excretory canals, infundibulum and oeso- 

 phagus. (Comp. PI. VII. Fig. 10). The flat epithelial ring above the apical organ is the 

 epidermis arching over the apical organ — not quite normally, due to the contraction on 

 preservation. The situation of the costse within the body in figs. 7— 9 is only apparent; 

 a comparison with the horizontal sections in PI. IX easily explains how this arrangement 

 has arisen through the intercostal parts arching over the deep lying costas. 



— 10. The apical organ of an embryo in stage III; seen in side view, from the sagittal side. 



Above the otolith is seen the polar field. ^20^^ 



— II. The same as fig. 10 seen from above; showing one excretory pore, the other being indis- 



tinct in the preparation. ^^°j\. 



