THE EDWARDSIA-STAGE OF LEBRUJflA. 315 



appearances of the coelomic spaces, d, e, f, g, present the successive 

 appearances from above downwards, showing how the single 

 mesenterial space ends in two distinct cornua. Between a, b the 

 separating mesentery has broken down and the two chambers are in 

 communication one with the other, and a little lower also with the 

 space in h. 



Fig. 13. The figure is made up of the appearances presented by several oblique 

 sections through the region of the termination of the archenteron. 

 Four radiating canals communicate with the two coelomic sinuses, 

 the latter separated by the first pair of mesenteries — sulco-lateral, 

 developed to a greater extent than the three other paii-s. The 

 limiting layer of the sinuses is both somatic and splanchnic. 



Fig. 14. A little below the archenteric region. The middle of the larva is 

 filled with vacuolated tissue; towards the upper side the coelomic 

 canals are completely separated one from the other, while on the 

 lower side the last traces of the connections are seen, and to the 

 right the last trace of the sulcar canal. 



Plate 19. 



Fig. 15. Vertical radial section of a freshly-extruded larva such as would be 

 obtained along the line x, y in Eg. 12. X 120. 



Figs. 16-20. Successive transverse sections through one of the non-extruded 

 larvse where disorganization of the central tissue is in progress. The 

 sections are such as would be obtained along the lines 1-1, 2-2, 3-3, 

 4-4, 6-6, respectively in fig. 21. X 120. 



Fig. 16. Section through the upper arclienteric region. The mesenterial 

 chambers are founded on a radial plan, four large and four small. 



Fig. 17. Section towards the termination of the archenteron. In the ventral or 

 sulcar sinus the splanchnic portion of the centi-al tissue shows two 

 separate regions of mor« deeply-staining tissue continuous with the 

 archenteric lining and with the limiting layer, while in the sulcular 

 sinus the reflected ectoderm is an almost continuous limiting 

 layer. 



Fig. 18. Section nearer the termination of the archenteron than in the last 

 figure. Owing to its reflection the lining of the archenteron is cut 

 through twice. The lumen of the archenteron is very small and 

 circular. X 320. 



Fig. 19. Section below the archenteron. The splanchnic portion of the 

 limiting layer and the tissue enclosed by it have become disorganized, 

 and only loose fragments remain. The somatic layer and the 

 vacuolated tissue between it and the supporting lamella are as yet 

 unchanged. 



