\S( 1DIACEA FORMATION OF ORGANS IN THE BUD. 



469 



*J 



disappears (Fig. 238 A'), while the epicardial sue (ep) can be followed 

 forward to the point at which its wide paired aperture (Fig. 238 A) 

 enters the pharynx (branchial 



sac). In other words, the epi- _ ' 



cardial sac arises from paired 

 apertures situated ventrallv to 

 the entrance of the oesophagus 

 and extends backward, its forked, 

 blind end becoming applied to 

 the pericardia] vesicle. The 

 latter is continued direct into 

 the stolonic septum. At an 

 earlier stage, the forked end of 

 the epicardium is found to be 

 continuous with the wall of the 

 pericardial vesicle, and the cavity 

 of the epicardium of the bud 

 communicates at this point with 

 the pericardial cavity. Conse- 

 quently, the cavity of the 

 stolonic septum, the pericardial 

 sac and the epicardium are 



merely differentiated portions of one and the same system of cavities. 

 In the larva, however, according to van Beneden and Julin, the 

 relation of the parts is different (p. 368).* 



With regard to the further development of the bud, we can merely 

 briefly mention that the gill-clefts appear as perforations of the 

 contiguous walls of the pharyngeal sac and the peribranchial sacs ; 

 that the muscles are derived from strands of mesenchyme-cells, and 

 that the branchial and atrial apertures owe their origin to ectodermal 

 invaginations which become connected with the pharynx and the 

 atrium. The endostyle appears in the form of a fold of the ventral 

 wall of the pharynx. 



FlG. li-iO. — Later ontogenetic stage of a 

 so-called bud of Amaroucium (after 

 Kiiwalevsky). <•/, atrium; (/.alimentary 

 canal ; e, atrial aperture ; ep, epicardium ; 

 es, endostyle ; i, branchial aperture ; ks, 

 gill-clefts ; n, nervous system ; p. peri- 

 branchial cavity. 



* [Julin (No. XVII. | has recently investigated the origin of the pericardium 

 in Distaplia, and finds a condition which differs somewhat from that described 

 above as occurring in Clavelina. As in Clavelina, a pair of epicardial (pro- 

 cardial) tubes grow out from the pharynx on either side of the oesophageal 

 aperture : of these the left becomes divided transversely into an anterior 

 portion, which then fuses with the left epicardial tube to form the epicardial 

 sac, and a posterior portion which gives rise to the pericardium and heart. 

 Prom the recurved end of the right sac the primary bud is given off before 

 the tube fuses with that on the left to form the median epicardial sac. — Ed.] 



