99 



nearly tVoiitally. Fig. 172 cuts the free end of the stalk tangentially, so as to shew a thick 

 mass of ectoderm (d.) whicli is the edge of the sucker. In this mass are seen the third body- 

 cavities of two l)uds [ó.\ ór) of different ages, in each case subdivided by a median mesentery. 

 The middle line of the actual anterior edge of the sucker corresponds with the interval between 

 the two buds, as is seen in sections which cut the parent stalk nearer its central axis. Some 

 of these sections demonstrate the continuit\- between the cavities of the buds seen in fig. 172 

 and that of the old stalk. 



Fig. 169 shews the highly crumpled condition of the ectoderm and of the basement- 

 membrane, due to the contraction of the muscles of the stalk. Part of the edge of the sucker 

 is still seen as two masses of ectoderm in contact with the younger bud [ó?]. In both the buds, 

 the edge of the mesentery which is further from the parent stalk corresponds with the anterior 

 surface of the bud. The older bud [è.') is cut somewhat obliquely through its stalk, the part 

 which is nearer its base being still indicated by the third body-cavities. 



The structure of the vouneer bud, which is cut in a direction at riijht ansfles to its long^ 

 axis, mav be studied from before backwards. Fig. 166 shews two cavities, lined by a distinct 

 epithelium, of which the smaller, the pericardium (pet'.), is more dorsally placed. The larger [è. c}) 

 is of course the anterior body-cavity. The pericardium is absolutely median, so far as can be 

 seen, although it has not been found possible to decide whether it is a median structure or not 

 in the younger stages. A section corresponding with fig. 166, though somewhat older, is figured 

 by M'IxTOSH (87, p. 28, PI. V, fig. 3), although the pericardium, which is marked er., is supposed 

 to be part of the collar-cavity. 



Fio-. 167 cuts the bud at the level of the collar-cavities and of the mouth (w/.) which is 

 overhung by the ventral lobe of the proboscis. In the next section, the alimentary canal appears 

 as a single vesicle, while in the next (fig. 168) the centrally placed nuclei {al.) belong to the 

 enteric sac, while the cavities at the sides {6. c.^) are the third body-cavities. While the inner 

 limit of the epithelium of these cavities is very distinct, the outer outline is much less so, and 

 I have not been able to identify with certainty the boundary between the antero-ventral ectoderm 

 and the internal tissues of the bud. Some of the thick ventral wall (upper in figures), both of 

 this section and of the section of the same bud {ó}) shevvn in fig. 169 may really belong to 

 the coelomic wall, and there are appearances in the sections which suggest that a proliferation 

 of cells takes place on the outer side of the coelomic epithelium, in the anterior and antero- 

 lateral regions, to give rise to the musculature of the body and stalk. The alimentarj' canal 

 has disappeared in fig. 169, although the median mesentery persists as far as the junction 

 with the parent stalk, both in this and in the older bud (figs. 169, 172). The mesentery seen 

 in fif. 169 (vounger bud) appears to contain a blood-vessel, while as another indication of a 

 probable vascular space may be noticed the interval shewn in figs. 167, 168 between the 

 epithelium of the alimentary canal and the coelomic epithelium. 



Fig. 170 is a section transverse to the principal axis of the older bud shewn in fig. 169. 

 The alimentary canal is in contact with the posterior body-wall, and the intestine (?w/.) is being 

 constricted oft' from the stomach {stoDi.). In the section (fig. 171) of the same bud next in front 

 of fig. 170 the intestine has been completely constricted oft", and it fills up the interval between 



