WITH NOTES ON THE WEST INDIAN SPECIES. 317 



are genetically related to the epiphysial complex of Craniota; in the crucial nuchal region 

 of the Enteropneusta are therefore to be found not the actual but the nearest possible 

 approximation to the actual primordia of the hypophysis cerebri and of the epiphysis 

 cerebri of Craniota. 



V. Genital Pleurae. 



From the statements and quotations contained in the preceding section it will be 

 seen that there is considerable consensus of opinion in regard to the definition of the 

 collar nerve-cord as the closed-in anterior portion of the dorsal trunk. 



Just as the medullary tube of the collar is admittedly an invaginated portion of the 

 dorsal nerve-trunk so the medullary folds which arise and fuse to form the medullary tube 

 are to be regarded as specialisations if the anterior portion of pleural folds which are 

 retained in the l'tychoderidae as the genital pleurae 1 . 



In the Craniota there are two principal methods of formation of the medullary 

 tube, namely ; — 



(1) By medullary folds as in Elasmobranchii, some Ganoids, Amphibia, Sauropsida 

 and Mammalia; 



(2) By solid proliferation or delamination as in Cyclostomes, some Ganoids, and 

 Teleostei. 



In the Protochorda we find essentially the same two methods in a simplified form, 

 namely ; — 



(1) Medullary folds in Urochorda; 



(2) A peculiar epithelial delamination in Cephalochorda. 



In the Enteropneusta, within the limits of the group itself we find the same two 

 methods, namely ; — 



(1) Medullary folds as in the Tornaria of Pt. biminiensis (Morgan 2 ) and in re- 

 generation of Pt. flava (above, p. 24o); 



(2) Delamination as in Bal. kowalevskii (Bateson). 



young Pt. minuta; and the remarkable keel also described by Spengel in adult Bal. kowalevtkii, etc.) can be 

 brought into accordance with the following definition. 



The median roots of the, Enteropneusta have arisen as differentiations from the raphe produced by the 

 fusion of the medullary folds over the cerebral portion of the dorsal nerve-cord to form the medullary tube 

 of the collar. This definition is based on the facts of anatomy. According to Morgan's account of the 

 development of the Birnini Tornaria (1894 loc. cit.) it is not borne out by the facts of development. 

 Singularly enough it does seem to be borne out in a remarkable manner in regeneration, which often appears 

 to point the way to a conception where ontogeny fails (see above, p. 246). 



1 Cf. section on Regeneration in Pt. flava, above, p. 245, and Figs. 5^1 — E on PI. XXVI. 



- This is all the more noteworthy because Pt. biminiensis (see above, p. 291) is one of those species 

 whose medullary cord does not possess an axial canal in the adult. The method adopted in Tornaria agassizii 

 as described and figured by Morgan is also the method of fusion of medullary folds, but by a strange 

 fatality my friend characterises it as being "exactly the same way that the nerve chord of Amphioxus is 

 formed" (T. H. Morgan, "The growth and metamorphosis of Tornaria," Jouni. Morph., Vol. v. 1891, see 

 p. 422). 



