292 ME. J. E. DUEKDRN OS 



5. Mesenterial Filaments. 



MeseBterial filaments are apparently undeveloped in the 

 youngest larvae, in wliich. the vphole of the vacuolated tissue is 

 intact. In the earliest stages, how^ever, in which disorganization 

 is in progress, a filament occurs on the first pair of mesenteries, 

 the sulco-lateral. These mesenteries, as already described, 

 remain connected with the oesophagus as far as its lower termi- 

 nation, are much larger than the three other pairs, and are very 

 prominent objects in transverse sections below the oesophagus 

 (PI. 19. figs. 19, 21). 



Of the filament only the middle Drlisenstreif or glandular 

 streak is yet formed. This structure stands out very conspi- 

 cuously in sections stained in carmine, by reason of the deeply- 

 staining character of its numerous close, oval nuclei, while the 

 free surface is also strongly ciliated. Long, narrow nematocysta, 

 similar to those in the superficial ectoderm and oesophageal 

 lining, occur here and there in different stages of development, 

 and occasionally granular gland-cells can be distinguished. 



The filamental tissue extends for some distance down the edge 

 of the mesenteries, but ceases before the aboral termination 

 is reached. Although probably a matter of no particular signifi- 

 cance, the filament during these early stages is found to be 

 rarely equally developed on each of the two mesenteries, and 

 generally terminates on one at a level different from that of the 

 other. 



In one larva sectionized transversely, the sulcular and sulculo- 

 lateral mesenteries, in addition to the sulco-laterals, remain 

 connected with the oesophagus as far as its aboral ending, and 

 throughout their middle free course are proportionately larger 

 than in other larvae and also than the sulcar directives. The 

 epithelium on each face is swollen a little, and towards the free 

 edge stains more intensely than elsewhere, indicating that 

 mesenterial filaments are in process of development. 



While in the oldest extruded larvae, and in all the non-extruded 

 specimens (fig. 21), tlie filamental tissue is in absolute continuity 

 with the similarly deeply-staining lining of the oesophagus or 

 archenteron, a break occurs in earlier stages of the first series, 

 where disintegration has just been set up. 



A distinct bridge of unmodified tissue is seen to intervene 

 between the oesophageal lining and the portion of the filament 



