460 



TUNICATA. 



according to Della Valle, is derived from a simple outgrowth of 

 the body-wall (consisting of ectoderm and the parietal layer of the 

 wall of the peribronchial sac), and thus arises in the way described 

 above for the buds of the Botryllidae. Jourdain (No. 64) has 

 recently stated that these two buds (the thoracic and the abdominal 

 buds) arise through the division of an original single bud, and that 

 the connection between the two halves is still maintained for some 

 time. No further details, however, are known on this point. Giard 

 (No. 58) holds that the first rudiment of the bud can here also be 

 traced back to the epicardial tube. 



The abdominal Imd (Fig. 232 A, k) is thus at first apparently an 

 outgrowth of the oesophagus of the parent which, however, soon 

 becomes more sharply marked off (Fig. 231 A and B) in such a way 

 that it then forms a caecum connected with the oesophagus only at 

 its anterior end (Fig. 231 G') ; this becomes U-shaped and can now 

 be recognised as the rudiment of a new intestinal loop, in which the 



A ,\J 



oc 



K ... 



«... 



X — 



Fig. 231. — Three ontogenetic stages of the oesophageal bud of Trididemnum* (after 

 Dei. i. a Valle), i, intestine; /'. intestine of tin- bud; /.■, bud; m, stomach; oe, 

 oesophagus; x, constricting ectoderm -ring. 



various parts (oesophagus, stomach, intestine) become differentiated. 

 The oesophagus of the newly-formed intestinal loop is connected with 

 the oesophagus of the parent (oe). The intestine of the bud becomes 

 applied to that of the parent (Fig. 232 A', //) and enters into com- 

 munication with it. The parent-individual now possesses two fully 

 developed intestinal loops, with continuous lumina. It is not yet 

 clearly known how the heart and the genital rudiments of the bud 

 develop, l>ut Della Valle believes that the latter are perhaps 

 derived direct from the genital rudiments of the parent. 



At the same time, the thoracic bud (Fig. 232 A, k') also develops 

 fully (Fig. 232, 11). We cannot here enter in detail into the some- 



* [The genus Trididemnum is included by Herdman in the genus Didemnum 

 Savigny. — Ed.] 



