96 



as fillino- the third body-cavity of this region. The young bud is occupied by a derivative ot 

 this packing tissue ; and although the cells in question may perhaps at first constitute a soHd 

 mass, they have arranged themselves as an epitheHum surrounding a central cavity at the 

 earliest stages recognised as young buds. The central cavity is in communication with the virtual 

 cavity of the parent stalk; and although this communication is interrupted in the particular 

 section figured (fig. 1 64), it is clearly seen in other sections of the same bud, and it is visible 

 in the older bud {è~) shewn in the same figure. The bud thus originates as a diverticulum of 

 the third body-cavity, which shortly gives rise to a corresponding projection of the ectoderm. 

 At its first appearance, as shewn in transverse sections, the diverticulum appears to be undivided. 



At a slightly later stage (fig. 162), as stated by Mastermax (98, 2, p. 515), the cavity 

 of the bud is divided by a prolongation of the anterior mesentery of the parent stalk. The 

 parent mesentery cafries a blood-vessel (a.v.), and although I have not been able to demonstrate 

 the entry of a branch of this vessel into the bud I have little doubt that Masterman is right 

 in stating that this is the fact. I cannot, however, agree with him that the three divisions of 

 the coelom remain continuous with one another up to the late stages shewn in his figs. t,"/ and 

 39. On the contrary, I find that in stages in which the ectoderm of the bud forms merely a 

 hemispherical outgrowth of the parent stalk (figs. 161 — 163) a terminal coelomic vesicle {^.c}) 

 is sharply marked off, and occupies the distal end of the bud. 



Figs. 161 — 163 cut the parent stalk transversely to its long axis, and the bud is 

 accordingly cut in a frontal direction. The three figures represent consecutive sections, fig. i6i 

 being the one which passes nearest the base of the parent stalk. The lateral origin of the bud 

 is clearly shewn by these figures. The proximal part is divided by a median mesentery, the 

 rio-ht and left divisions of the coelom communicating freely with the parent stalk. The distal 

 end of the bud in fig. 1 6 1 is occupied by a spherical vesicle, with a well marked coelomic 

 epithelium. In the next section (fig. 162) the vesicle is divided from the paired coelom of the 

 right side of the figure by a minute oval vesicle [per?.], which contains several nuclei. In the 

 next section (fig. 163) the left paired cavity has almost disappeared, while the distal end of 

 the bud is occupied by a cavity whose wall is not certainly constituted by an epithelium. I am 

 unable to decide whether this cavity is continuous with the terminal cavity of fig. 162 or whether 

 it is to be regfarded as a vacuolated condition of the terminal ectoderm ; but I am inclined to 

 adopt the latter hypothesis. It is in any case quite clear that in the young bud of C. gracilis 

 shewn in PI. III, fig. 26, the end of the bud is occupied by a single ovoid epithelial vesicle. 



Although I cannot identify the parts of the young bud shewn in figs. 161 — 163 with 

 certainty, it appears to me highly probable, from a comparison with later stages, that the vesicle 

 marked b. c} is really the coelomic sac of the anterior body-cavity, that the paired cavities 

 represent the third body-cavities and probably the collar-cavities as well, and that the minute 

 structure per?, is the pericardium. There is no clear evidence to shew how these structures 

 have originated '), though I consider it probable that they have been formed by the division 

 of the single coelomic .sac seen in the younger Inul in fig. 1 64. It is not impo.ssible that the 



l) In regenerating specimens of lialanoglossus, Dawydofi- (02) linds that the pericai'dium is deiived from the proboscis-cavity, 

 nd the hitter fiom the pevihaemal spaccs; the amputation haviug been made thiuugh the collar. 



