Plate IX. 



Figs. 1—5. Successive horizontal sections through an embryo in stage II. The entoderm too badh' 

 preserved to be figured (the large vacuolated cells apparently fill the whole space 

 between the ectoderm and oesophagus, but an entodermal cavity is doubtless present. 

 Figs. 1—3 show the costse, 2 — 3 the tentacle-rudiment. In fig. 2 the oesophagus is cut at the 

 level, where it passes into the transversal furrow; the latter is cut at different le\-els in 



figs- 3-5- %■ 

 — 6—14. Successive horizontal (slightly oblique) sections through an embryo in stage III, prepared 

 out of the egg-membrane. ^°/i. 



Fig. 6 goes through the upper edge of or above the apical organ, showing only the 

 excretory canals. The epithelial lining around them is the epidermis, the apical organ 

 having been rather sunken in the specimen sectioned. The substance lying between the 

 two excretory canals is the cupule. (Comp. PI. VII. Fig. 3). 



Fig. 7. The section goes through the apical organ. (Comp. PI. VII. Fig. 8). 



Fig. 8. The section just touches the lower side of the apical organ (to the left side, the 

 direction being slightly oblique), otherwise it goes through the infundibuluni. To the 

 right the tentacle apparatus begins to appear. 



Fig. 9. The section goes through the upper end of the oesophagus. The right tentacle 

 apparatus is seen. 



Figs. 10 — 12 show the oesophagus, the four entodermal sacs and the tentacles. 



Fig. 13 is at the level, \vliere the oesophagus passes into the transverse furrow, fig. 14 

 passes through the furrow, below the oesophagus. The right tentacle is seen lying 

 within the furrow. In figs. 6 — 12 are seen the costse, lying in their furrows, the inter- 

 costal parts arching somewhat over them. 



(The fig. 12 has perhaps become a little too large, belonging to the same section as the 

 other figures; the reason is that the epidermis of the specimen has wrinkled to some 

 extent; but these wrinkles, doubtless exclusively due to the preservation, have been elim- 

 inated and the natural shape reconstructed. The same has been the case with all the 

 sections, of embryos as well as grown specimens, the artificial wrinkles having every- 

 where been left unreproduced, the natural course of the ectodermal line having been 

 reconstructed). 



