246 EXTEROPXEUSTA FROM THE SOUTH PACIFIC, 



In possessing, in such a high degree, the faculty of regeneration, the Enteropueusta 

 differ radically from Amphioxus, which does not regenerate after fracture. 



The possibility of regeneration revealing facts of atavistic significance is a matter 

 of great suggestiveness. At the same time, the assertion that any particular process 

 of regeneration is atavistic is always liable to be dismissed as arbitrary. It is very 

 important to bear in mind that regeneration at different regions of the body may be 

 accompanied by different appearances and will yield different information. The atavistic 

 phenomena associated with regeneration carry us farther back than do the phenomena 

 of ontogenetic recapitulation. Although Morgan has found that in Tornariu the collar 

 nerve-cord arises by the depression and closure of a medullary groove, yet it could 

 not there be recognised that the medullary folds are metamorphosed derivatives of pre- 

 existing pleural folds. It is practically certain that Pt. flava develops through a Tornaria 

 stage. No Tornaria has ever been seen having two water-pores. On the other hand 

 in a regenerating Pt. flava we find a restoration of what must have been the primordial 

 condition, viz. equal paired proboscis- pores. 



In the regenerating individual shown in Fig. oA, PI. XXVI., the medullary folds 

 are seen to be widely separated in front and less widely separated behind (cf. PI. 

 XXXII. Figs. 66—67); in Fig. 5B the medullary folds are closely approximated and 

 transverse sections reveal the fact that they are actually fused together over a short 

 stretch at the extreme anterior end of the collar (PL XXXII. Fig. 6S) behind which 

 they are still unfused, the narrow superficial groove leading directly into a wide medul- 

 lary canal. At this and at the preceding stage there is no free collar-flap in front. 

 In the stage of regeneration shown in Fig. 5D a median dorsal groove is seen to 

 occupy the posterior two-thirds of the collar region, and in front of the groove is a 

 smooth tract which represents the anterior free collar-flap 1 . 



After the closure of the medullary folds the collar continues to grow in length 

 and to project forwards as a free circular fold (collar-flap). The medullary tube must 

 also be involved in the general growth in length and so we find that it extends 

 forwards in front of the dorsal septum which, as in the normal adult, has an anterior 

 free border. Thus although in this specimen (Fig. 5D) the dorsal septum is not 

 coextensive anteriorly with the medullary tube, it is so posteriorly and it presents 

 clearly the appearance of resulting from and representing the raphe of fusion of the 

 medullary folds 2 . Inside this dorsal septum there are cellular remains of the solid plate 

 or keel of ectoderm which is produced by the fusion of the folds (cf. PI. XXXII. 

 Fig. 68 and Text-fig. 1). Sometimes these remains are in the form of disconnected 



1 Reference is made below to the anterior " Epidermistasehe " which Spengel describes in place of the 

 anterior neuropore. In the specimen of Pt. flava shown in Fig. 5 D there is no question of an " Epi- 

 dermistasehe " which in other cases may coincide with the neuropore. In this case there is only the true 

 neuropore at the anterior end of the fused medullary folds. The collar-flap projects above and beyond it, 

 but there can be no confusion between the angle formed by the collar with the neck of the proboscis and 

 the neuropore, such as is possible in certain cases. 



- The dorsal septum which, when present, unites the collar nerve-cord with the epidermis, should not be 

 confounded with the dorsal mesentery which primarily separates the two collar-cavities from one another at 

 an early stage. This mesentery is represented in the adult by the median partition between the right and 

 left perihaemal cavities, which carries the dorsal blood-vessel. 



