218 ARTHUR WILLEY. 



(see Balfour, ' Comp. Emb./ vol. ii), into which the ueurenteric 

 canal opens ; but there is every probability that there is no 

 more post-anal gut in the Ascidian embryo than there is in 

 the embryo of Amphioxus. 



The mesoblast consists of two longitudinal bands differen- 

 tiated from the primitive hypoblast. 



These bands were formerly thought to be quite solid, but van 

 Beneden discovered (loc. cit.) that anteriorly they occur as a pair 

 of archenteric pouches (see Woodcut, fig. 1). The mesoblastic 

 bands in the region of the so-called tail are solid, and consist of 

 only a single layer of cells continuous ventrally with the remains 

 of the hypoblast ; but in front they consist of several layers of 

 cells, which at first surround a short lumen which communi- 

 cates, as stated above, with the enteric cavity on each side. 



This single anterior pair of openings into the alimentary 

 canal is the only indication in the Ascidians of archenteric 

 pouches, and it is regarded by the authors as equivalent to the 

 first pair of somites of Amphioxus. The rest of the mesoblast 

 in Ascidians is unsegmented, and the general absence of meso- 

 blastic pouches, with the exception of the first pair, is con- 

 nected with the early atrophy of the posterior or subchordal 

 portion of the alimentary canal, and is therefore probably 

 a product of degeneration, and not an ancestral character. 



In the embryo of Amphioxus, at a roughly corresponding 

 stage (Woodcut, fig. 2) the notochord does not reach the 

 anterior end of the body, but even at this early period it extends 

 beyond the anterior opening of the nerve-tube (cf. Woodcut, 

 fig. 1). Partly in front of the notochord and partly below it 

 is a portion of the alimentary canal which lies in front of all 

 the mesoblastic somites. This anterior chamber consists of a 

 median portion and two lateral horns. The latter become con- 

 stricted off from the chamber, and form the anterior intestinal 

 diverticula, whose entire development has been fully described 

 by Hatschek(3); while the median portion of the chamber 

 simply merges with the anterior extremity of the alimentary 

 canal, and ceases to be recognisable as a distinct chamber. As 

 is well known, the left anterior diverticulum opens to the exterior 



