288 ME. J. E. DUEEDEN ON 



The internal appearances of the various regions of a later 

 larva, one in wliich the disorganization of the central tissue is in 

 progress, though the mouth is not formed, are represented in 

 PI. 19. figs. 16-21. The first five are taken from actual sections of 

 one of the non-extruded polyps, while fig. 21 is constructed so as 

 to represent what would be obtained along the lines 1-x, x-1 in 

 fig. 19. As a matter of fact, however, fig. 21 was obtained 

 almost entirely from a single vertical section. The sections from 

 which figs. 16-20 are taken would be along the lines 1-1, 2-2, 

 3-3, 4-4, 5-5 respectively of fig. 21. 



Fig. 16 passes through the upper region of the archenteron, 

 just below the sections showing the sixteen separate slit-like 

 cornua. The spaces are very regular and crescent-shaped. 

 Pig. 17 approaches the inner termination of the archenteron, 

 only the sulco-lateral pair of mesenteries being still attached to 

 the latter. The sulcular and sulcar sinuses are established; but 

 neither here nor lower is there any trace of the archenteron 

 dividing into canals. Pour regions are indicated on its ccelomic 

 epithelium, in which the deeply-staining tissue in continuity with 

 the arch enteric lining is becoming concentrated. The two in 

 the sulcar sinus are distinct, but the sulcular pair are nearly 

 united, though more distinct in sections above. 



The appearance of the terminal walls of the tube alone is 

 represented in fig. 18. The deeply-staining endoderm is cut 

 through twice as a result of its reflection (cf. fig. 21, 3 — 3). 

 The reflected moiety is now separated into two parts by the con- 

 nection of the sulco-lateral mesenteries, which are seen to extend 

 as far as the actual termination. The same tissue, developed to 

 a much less degree, can also be traced along the margin of the 

 mesenterial epithelium. 



The next figure (fig. 19) is from a section below the archenteron, 

 and shows the larger, filament-bearing, sulco-lateral mesenteries^ 

 and relics of the disorganized central tissue, in the gastro- 

 coelomic cavity now m progress of formation. Comparing this 

 with figures 9-11 of the earlier stage, it will be seen that the 

 portion to disintegrate is that central to the two sinuses, including 

 both the limiting layer and the enclosed mass of parenchymatous 

 tissue. 



The peripheral portion has not yet commenced the changes 

 resulting in the unilaminar epithelium of the adult ; it is still 



