UROCHORDA. 15 



entiatioii of pai-ts of the cells is still doubtful. Kupffer inclines to the lat- 

 ter view : the analogy of the notochord of higher types appears to me to tell 

 in favour of the former one. 



The alimentary tract. The anterior part of the primitive ar- 

 chenteron alone retains a lumen, and from this part the whole of the 

 permanent alimentary tract (mesenteron) becomes developed. The 

 anterior part of it grows upwards, and before hatching an involu- 

 tion of the epiblast on the dorsal side, just in front of the anterior 

 extremity of the nervous system, meets and opens into this upgrowth, 

 and gives rise to the permanent mouth (fig. 8 V. o). 



Kowalevsky states that a pore is formed at the front end of the nervous 

 tube leading into the mouth (tig. 8 v. and vi. y*) which eventually gives 

 rise to the ciliated sack, which lies in the adult at the junction between the 

 mouth and the branchial sack. Kupifer however was unable to find 

 this opening; but Kowalevsky's observations are confirmed by those of 

 Salensky on Salpa. 



From the hinder end of the alimentary sack an outgrowth directed 

 dorsalwards makes its appearance (figs. 8 V. and 9, d), from which the 

 oesophagus, stomach and intestine become developed. It at first 

 ends blindly. The remainder of the primitive alimentary sack gives 

 rise to the branchial sack of the adult. Just after the larva has be- 

 come hatched, the outgrowth to form the stomach and oesophagus, etc. 

 bends ventralwards and to the right, and then turns again in a dorsal 

 and left direction till it comes close to the dorsal surface, somewhat to 

 the left of and close to the hinder end of the trunk. The first ventral 

 loop of this part gives rise to the oesophagus, which opens into the 

 stomach; from this again the dorsally directed intestine passes off. 



On the ventral wall of the branchial sack there is formed a narrow 

 fold with thickened walls, which forms the endostyle. It ends ante- 

 riorly at the stomodseum and posteriorly at the point where the solid 

 remnant of the archenteron in the tail was primitively continuous with 

 the branchial sack. The whole of the alimentary wall is formed of a 

 single layer of hypoblast cells. 



A most important organ connected with the alimentary system 

 still remains to be dealt with, viz. the atrial or peribranchial cavity. 

 The first rudiments of it appear at about the time of hatching, in 

 the form of a pair of dorsal epiblastic involutions (fig. 8 v. kl), at the 

 level of the junction between the brain and the spinal cord. The^e 

 involutions grow inwards, and meet corresponding outgrowths of the 

 branchial sack, with which they fuse. At the junction between them 

 is formed an elongated ciliated slit, leading from the branchial sack 

 into the atrial cavity of each side. The slits so formed are the first 

 pair of branchial clefts. Behind the first pair of branchial clefts a 

 second pair is formed during larval life by a second outgrowth of the 

 branchial sack meeting the epiblastic atrial involutions (fig. 8 vi. Iks 

 and 2^'s). The intestine at first ends blindly close to the left atrial 

 involution, but the anus becomes eventually formed by an opening 



