470 TUNICATA. 



< Ontogenetic processes altogether similar to those just described are 

 found in cases where the detached parts of the post-abdomen in the 

 PolycUnidae regenerate (Fig. 239). Here also the central entoderm- 

 vesicle is the first seat of transformation. As has already been men- 

 tioned (p. 454), the first change to occur is a widening of the proximal 

 end of the epicardial septum (Fig. 239 B). An entoderm-vesicle is 

 in this way formed in the proximal part of the young individual, and 

 this is continued backward into the part of the stolonic septum which 

 did not widen. The proximal dilatation is the rudiment of the whole 

 alimentary canal of the new individual, while from the non-widened 

 part are derived the epicardial sac (Fig. 239 B, ej>) and probably 

 also the pericardium. The entoderm-vesicle here also divides up in 

 the same way as in the bud into three lobes (Fig. 239 A and B), the 

 middle lobe being the rudiment of the branchial sac and the two 

 lateral lobes the rudiments of the peribronchial sacs. The complete 

 abstriction of the latter, their interconnection (Fig. 239 C) to form 

 a median impaired dorsal part (atrium), and the development of the 

 gill-clefts all occur in the same way as in the formation of buds. 

 The rudiment of the alimentary canal is here also a small, unpaired 

 caecum (Fig. 239 B, </), which grows out at the dorsal side in the 

 posterior part of the entoderm-vesicle and curves into the shape of 

 the letter U. In the development of the more important systems 

 of organs, we thus have here complete agreement with the processes 

 of gemmation. 



2. Doliolidae. 



We have already mentioned (p. 388) that two stolons are apparently 

 found in the " nurse " (blastozooid) generation of Doliolum, one ventral 

 (us) and the other dorsal ('/.«••), the dorsal outgrowth developing to a 

 much greater size than the true ventral stolon, and finally giving rise 

 to heteromorphous individuals known as lateral and middle buds 

 (the gastrozooids and phorozooids). The two stolons vary greatly in 

 structure. As will be seen later, the so-called dorsal stolon is merely 

 a dorsal outgrowth of the body which receives and gives attachment 

 to the buds derived from the true ventral proliferating stolon, hence 

 it is better to restrict the term stolon to the ventral structure and to 

 speak of the dorsal structure as the dorsal outgrowth. 



The ventral stolon (Figs. 180, /•, p. 385; 2-13, /•; 2-15 A, vs), formerly 

 known as the rosette-like organ, and first recognised by Grohben ( No. 

 79) in its true character as proliferating stolon, appears as a pear- 



