WITH NOTES ON THE WEST INDIAN SPECIES. 245 



REGENERATION IN PT. FLAY A. 



Like Pt. minuta (cf. Spengel, Taf. XXVI. Figs. 14 — 18) Pt, flava possesses extraordinary 

 powers of regeneration, and the processes involved in the regeneration of the proboscis 

 and collar are unusually instructive, especially if, as Barfurth 1 and others think, re- 

 generation is sometimes (not always) accompanied by atavistic phenomena. Of course 

 the phenomena of regeneration will vary according to the region in which it takes 

 place. For example, if it occurs behind the genital pleurae (PI. XXVI. Fig. 5 C) the 

 appearances presented are differenl from those which are exhibited when the regene- 

 ration occurs in the region of the pleural folds. It is these latter cases to which 

 I desire to draw particular attention. 



The chief facts observed are evident in the figures on Plate XXVI., and it will 

 suffice to point out the principal conclusions derived therefrom. I have confirmed 

 these conclusions as far as possible by sections, some of which are reproduced on 

 Plate XXXII. 



1. When regeneration occurs in the region of the genital pleurae the collar is 

 regenerated from the pleurae. 



2. The collar nerve-tube is formed by the fusion of true medullary folds which 

 are differentiated from the pleural folds (PI. XXVI. Figs. 5 A and 5 B). 



3. The zones of the collar are differentiated from the annulations of the body-wall 

 ( Fig. 5 E). 



4. In regenerating individuals the right and left proboscis-pores are approximately 

 equal. In the specimen shown in Fig. bA they were quite equal. 



5. In regenerating individuals the lumen of the stomochord is, at first, entire. 



For my part I am persuaded that the above facts have an atavistic significance. 

 I do not think there is any reason for regarding the collar as being anything more 

 than a differentiation of the anterior portion of the trunk associated with the cepha- 

 lisation and regional differentiation in general, which is such a prominent characteristic 

 of the Enteropneusta. This remark refers simply to the collar as such, and not to 

 the pair of body-cavities which form the collar-coelom. These cavities may possibly 

 date much farther back than the collar itself, which is a purely Enteropneustic structure. 



As MacBride 2 has shown, homologous coelomic pouches occur in Amphioxus where 

 there is no collar. It is important not to confound the collar with the collar-coelom. 



1 Dietrich Barfurth, " Regeneration und Involution " in Merkel u. Bonnet's Ergebnisse der Anat. u. Entwick- 

 gesch. Bd. iv. 1894. 



- E. W. MacBride, "The early development of Amphioxus," Q. J. M. S., Vol. xl. 1897—8, p. 589. 



