On the Formation of the Pelvic Plexus. 



445 



a relatively high resistance as compared with that of the albuminous 

 fluid filling the remainder of the space. It has been sometimes sug- 

 gested that alterations of resistance may play an important part in 

 the phenomena of organ activity. The experiments just given appear 

 to indicate that the discs have a resistance which is of a different order 

 to that of the physiological saline in the surrounding media ; but even in 

 the case of these protoplasmic structures the results scarcely warrant the 

 belief that there is anything exceptional in their higher resistance since 

 it only places them in the same category with such other excitable tissues 

 as muscle and nerve, which have been shown to offer a greater electri- 

 cal resistance in the transverse than in the longitudinal direction. 



On the Formation of the Pelvic Plexus, with especial Eeference 

 to the Nervus Collector, in the Genus Mustelus." By E. C. 

 Punnett, B.A., Scholar of Gonville and Caius College, Cam- 

 bridge. Communicated by Hans Gadow, F.E.S. Eeceived 

 June 30 — Eead November 16, 1899. 



(Abstract.) ' 



The main object of this investigation was to ascertain whether at any 

 period in the development of the animal selected, the number of 

 branches composing the nervus collector was greater than that found in 

 the adult. As a logical consequence of Gegenbaur's theory we should 

 expect such to be the case, and the ontogenetic history of the nervus 

 collector recorded in this paper, its maximum development in young 

 embryos, and its subsequent gradual decrease through the later stages 

 of embryonic existence leading to its condition in the adult, must, if 

 there is any truth in the recapitulation theory, all point to its primitive 

 character. 



The history of the posterior collector, the very existence of which 

 has not hitherto been described, throws important light upon the theory 

 mentioned above. Here we have a collector formed in the embryo, 

 from which in later stages the component nerves separate and run 

 singly into the fin. Such a fact points very strongly to the collector 

 condition being more primitive than that condition in which the nerves 

 reach it without previously effecting any junction with one another. 



It is further shown that the formation of this collector is clue to 

 migration of the whole fin rostrally, and not merely to a contraction of 

 the fin area, and in support of this the following evidence is brought 

 forward. The two species, M. Icevis and M. vulgaris, differ from one 

 another chiefly in the more rostral position of the pelvic girdle in the 

 former. That it is highly improbable such a condition should be due 



