ASCIDIACEA FORMATION OF ORGANS IN THE BUD. 



465 



the dorsal middle line (Fig. 236 /;, j>). These processes grow 

 towards each other and fuse, and thus the single atrial cavity arises 

 (Fig. 237, cl). In the meantime the two peribranchial sacs have 

 become completely disconnected from the central cavity. 



The peribranchial sacs arise in a slightly different manner in Peroph&ra 

 (KOWALEVSKY, HITTER), in which form, instead of separating as two distinct 

 sacs and fusing together at a later period, a single bilobed sac separates from 

 the inner vesicle ; thus the definitive atrial cavity is formed at an earlier 

 period, the development being apparently abbreviated. SEELIGER also states 

 that, in Clavelina,Baa unpaired vesicle becomes abstricted from the dorsal side 



FIG. 236. Two stages in the development of the buds of IHstaplia stylifera (after 

 KOWALEVSKY). A, younger, B, older stage, d, alimentary canal ; dr, digestive 



gland ; ./', rudiment of the stolon ; //. genital rudiments ; n, nerve-tube ; p, right 

 peribranchial sac. 



of the enteric sac, which persists as the atrium and, growing round the sides 

 of the pharynx, forms the peribranchial sacs. HJORT (No. 59) again, recently 

 stated that, during the budding of Botrtjllus, a saddle-shaped vesicle becomes 

 separated from the inner vesicle of the bud and gives rise to the atrial cavity 

 and the paired peribranchial sacs. 



At the time when the peribranchial sacs form, an unpaired caecum 

 grows out from the posterior end of the entoderm- vesicle (Fig. 236 

 A, d) ; this soon bends to the left and thus becomes the rudiment 

 of the intestinal loop (Fig. 236 B, d) in which latter the different 

 sections (oesophagus, stomach and intestine) can be more distinctly 

 made out. A diverticulum (dr), rising from the pyloric region, 

 develops into the rudiment of the so-called digestive gland (Fig. 237).. 



HH 



