PYROSOMA FURTHER DEVELOPMENT OF THE BUDS. 



493 



FIG. 260. Two early stages in the develop- 

 ment of the pericardial vesicle in the buds 

 of Pyrosoma (after SEELIGER). en, entoderm 

 (wall of the enteric rudiment) ; ec, ectoderm ; 

 p, peribranchial sac ; pc, rudiment of the 

 pericardial vesicle. 



tibrillae increase in size, they become band-like. In transverse sec- 

 tion, they then appear radially arranged, while the centre of the 

 muscle-bundle is occupied by 

 the undifferential protoplasm 

 and nuclei of the cells. 



The first rudiment of the 

 pericardial vesicle IB &\&o traced. 

 back, by SEELIGER, to a small 

 group of mesenchyme-cells 

 which can be seen beneath the 

 distal end of the right peri- 

 branchial tube. In this cell- 

 group (Fig. 260, y>c) a lumen 

 soon appears which is the 

 pericardial cavity ; at a later 



stage, the rudiment of the heart (hz) arises through the invagination 

 of that part of the wall of the pericardial vesicle which is in contact 

 with the wall of the intestine (Fig. 261). 

 The pericardial vesicle originally lies on 

 the right side of the bud, but wanders 

 later towards the median plane and thus 

 reaches the dorsal side of the endostyle, 

 process (Fig. 253, hz). It has therefore 

 in Pyrosoma, a position unlike that 

 occupied by it in the other Ascidiacea, 

 where, as described above (p. 370), the 

 heart lies on the ventral side of the 

 endostyle-process (epicardium) in such 

 a way that the epicardial lamella brings 

 about the closure of the cardial tube. 



In each bud, the genital strand (Fig. 

 259, g) gives rise to the genital organs 

 of the individual and to the genital 

 strand of the stolon produced by that 

 individual. At an early stage, the 

 genital strand is found to break up into 

 a distal and a proximal part. The 

 distal part, which is embedded in the 

 elaeoblast- tissue (eb), becomes abstricted 

 and gives rise to the genital strand of 

 t/ic stolon. In the proximal part, which represents the rudiment of 



FIG. 261. Two later stages in 

 the development of the peri- 

 cardial vesicle in the bud of 

 Pyrosoma (after SEELIGER). 

 ec, ectoderm ; el,, elaeoblast ; 

 en, entoderm (wall of intes- 

 tine) ; pc, pericardial vesicle ; 

 hz, rudiment of heart. 



