96 



as filliiij^f the third l>ody-cavil\- of this region. The \')ung bud is occupied by a derivative of 

 this packing' tissue-, and although the cells in question may perhaps at first constitute a solid 

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

 earliest stages recognisetl 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 {d') shewn in the .same figure. The butl 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. 



.\t a slightly later stage (fig. 162), as stated by Masterman (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 carries 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 1 liave 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. 37 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 ((5.^-') 

 is sharply marked oft", 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. 161 

 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 pro.ximal part is divided by a median mesentery, the 

 right and left divisions of the coelom communicating freely with the parent stalk. The distal 

 end of the bud in fig. 161 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 {/>er.}), which contains several nuclei. In tiie 

 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 b)' 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 regarded 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. Ill, 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. r.' 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 tt) shew how these structures 

 have originated '), though I consider it probable thai the)' have been formed by the tlivision 

 (if the single coelomic .sac seen in the younger l)ud in lig. 1O4. It is not impossible that the 



I) In regenerating specimens of Balanoglossus, DawyijOFF (02) tinds that the pericardium is derived from the proboscis-cavity, 

 and the latter from the perihaemal spaces; the amputation having been made through the collar. 



