of tlie Pelvic Plexus in Acanthias vulgaris. 19 



more rostral position of the girdle on the side-fold theory, we might 

 expect to find some trace of this in the number of post-girdle vertebrae 

 that is, we might reasonably look for a larger number of whole 

 vertebrae behind the girdle-piercing nerve when the girdle is situated 

 more rostrally than when it is more caudally placed. A glance at 

 Table XIII will show that this is only to a very small extent the 

 case. Whilst the girdle shows a difference in position amounting to 

 five segments, there is a difference of less than one between the 

 average number of post-girdle whole vertebrae associated with its 

 extreme positions. 



On the migration theory such difficulties are not encountered. The 

 comparative constancy in the number of post-girdle whole-vertebrae 

 for all positions of the girdle is to be explained by supposing that 

 there exists some relation between the position of the girdle and the 

 point where that more flexible portion of the body the tail with its 

 half-vertebrae commences. The position where the " Anlage " of the 

 pelvic fin is laid down must be supposed to determine, probably for 

 mechanical reasons (cf. Gadow (6), p. 195), the point where the half- 

 vertebrae shall start. Moreover, supposing the migration to be 

 secondarily in a rostral direction helps us to understand why the 

 number of post-girdle whole- vertebrae tends to be rather greater for the 

 most rostral positions of the girdle, than for the intermediate or most 

 caudal positions. We must imagine that the conversion of whole into 

 half-vertebrae tends on the whole to keep pace with the rostral migra- 

 tion of the girdle. As the migration of the girdle attains its maximum 

 the formation of half-vertebra? tends, so to speak, to lag behind, and 

 as a consequence we find that in the most rostral positions of the 

 girdle a rather larger average number of post-girdle whole-vertebra?. 



Some Embryological Data. 



The material used consisted of Acanthias embryos which had been 

 preserved in corrosive sublimate and acetic acid. Horizontal longi- 

 tudinal sections were cut through the pelvic area. These were 

 treated with gold chloride and formic acid as described in a previous 

 paper ((12), p. 344). Such portions of the plexus Avhere the inter- 

 communications of the nerves was required, were re-constructed on 

 millimetre paper. The results may be tabulated as follows : 



C 2 



