16 Mr. E. C. Punnett. On the Composition and Variations 



These tables show that, whilst the serial number of the girdle- 

 piercing nerve is identical, the average number of collector branches is 

 slightly less in the embryos than in the adults. 



Consequently, for practical purposes, such embryos may be con- 

 sidered as being of equal values with the adults. From analogy with 

 the case of Mustelus quoted above, it might have been expected that 

 these older embryos would have shown a slightly larger number of 

 collector nerves than the adults. It must, however, be borne in mind 

 that these embryos of Acanthias are rather more advanced than those 

 of Mastdus, which were previously considered. As far as I have been 

 able to observe, an Acanthia* embryo of a given length about this 

 stage is more advanced than a Mustelu* embryo of the same length. 



Eeturning now to Table VIII, if we are to explain the correlation 

 there expressed on the side-fold excalation theory, we must suppose 

 that the lesser number of collector branches associated with a more 

 rostral position of the girdle is due to excalation of vertebrae through 

 which pass nerves to the collector as well as vertebrae in the pre- 

 collector area. This will be made clearer by the consideration of a 

 concrete case. In fig. 6 two cases have been selected, one in which 



FirsG 



b. 



Coiiector &re& shaded. 



FIG. 6. 



the first girdle-piercing nerve is thirty-five, and the other in which it 

 is forty. From Table VIII. we learn that the average number of 

 collector nerves associated with these two positions of the girdle are 

 ten and twelve respectively. Now if we are to derive the case in 

 which the girdle is more rostrally situated from that in which its 

 position is more caudal, the figure shows that we must assume two 

 segments to have been excalated in the collector area, and three in 

 the pre-collector portion of the trunk. The assumptions made are 

 (1) that the girdle-piercing nerve is homologous in each case, and (2) 

 that a more rostral position of the girdle is in itself no argument for a 

 lessened collector area. Consequently we must assume that on the 

 side-fold excalation theory, excalation takes place both in the collector 

 and the pre-collector areas. 



