Page:Proceedings of the Royal Society of London Vol 69.djvu/11

Rh (ventraler Langsstamm) from which the nerves to the muscles are given off. The dorsal branches behave similarly, but the arrangement is slightly more complex owing to the fact that two such longitudinal nerve stems are found, the " medialer " and the " lateraler Langs- stamm."* The ventral branch of the 40th nerve passes through the more medially situated of the two pelvic foramina before joining the longitudinal stem. The dorsal and ventral branches of nerves 31 39 are indistinguishable along the greater part of their course, as they all run together in a long nerve stem the nervus collector until just before the point where the girdle is reached. Here the dorsal branches contained in the collector separate from the ventral branches. The former are continued as a branch which runs into the dorsal " Liingsstamm," whilst the latter pierces the more laterally situated of the two pelvic girdle foramina and enters the ventral " Langsstamm." The specimen described by Braus differs considerably, since (1) the nerves entering the fin are 25 45, (2) the girdle is pierced only by the 36th nerve which apparently corresponds to DavidofFs 40th nerve, and (3) the ventral branches of the collector do not pass through a foramen. These two cases alone are sufficient to show that a considerable amount of variation occurs.

The object of this investigation was to ascertain the range of such variations, and to endeavour to find out whether they afford any support or otherwise to either of the two theories which attempt to explain the varying position of the limbs in different species and in different members of the same species, i.e., the theory involving verte- bral inter- or excalation and the theory of limb-migration. For this purpose a number of individuals of both sexes was examined, and the following points in each specimen accurately determined :

(a) The serial number of the girdle-piercing nerves.

(b) The number of nerves forming the collector.

(c) The number of post-girdle nerves.

(d) The point of junction of the whole- and half-vertebra (" Ganz- und Halb-Wirbel ").

For the determination of the serial number of the nerves piercing the girdle the arrangement of the occipital region was in each case examined. This was found to be practically constant, thus affording a fixed point from whence to reckon. The nomenclature adopted for the nerves of this region is shown in fig. l.f

The determination of the point of junction of half- and whole


 * Cf. loc. cit., Taf. 29, fig. 15, and Taf. 31, fig. 27.

t This nomenclature has been adopted for convenience, and it does not neces- sarily follow that the nerve here denoted as z is homologous with the nerve similarly designated in other forms by Fiirbringer and others. As Fiirbringer's Memoir (8) is not accessible, I am unable to say at present whether for this species his z and my z are identical.