of the latter, or is slightly inclined towards its free tip; 

 Pig. 2. 



It novi represents the characteristic double vesicle 

 of the young Ascidian bud; the outer or ectodermal vesicle is 

 directly derived from the ectoderm of the stolon, and the 

 inner or entodermal vesicle, v/hich has become a closed sac, 

 arises by evagination of the thickened portion of the dilated 

 stolonic partition. The connection between the partition 

 and the inner vesicle is retained for a long time, and the 

 cavity of the latter is to be regarded as being in communi- 

 cation with the potential cavity of the partition. 



A transverse section at this stage, Fig. 8, shows 

 the still somewhat thickened cells of the ectoderm of the bud 

 rudiment, and the endodermal vesicle, lying inside, v;-i th its 

 much thickened v/alls. Active cell-multiplication has been 

 going on in the latter, which are of uniform thickness, ex- 

 cept below where they pass off gradually into the thin walls 

 of the partition. The cells of the blood are found scatter- 

 ed about betv/een the two vesicles, and it is to be especially 

 noted that many are lying close against the inner surface of 

 the ectodenn and outer surface of the endoderm at niimerous 

 points . 



The bud-rudiment does not long remain spherical, 

 but soon becomes elongated by a grovTth towards tne apex of 

 the stolon. It now assumes an oval shape, and one end lies 

 free over the surface of the stolon beyond the portion which 

 is directly connected with the latter, Pig. 3. It is the 

 free end which is later to be distinguished as the anterior 



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