Page:Proceedings of the Royal Society of London Vol 69.djvu/13

Rh centrum we suddenly meet with a centrum over which there issues no spinal nerve (cf. fig. 2, centrum 49). This centrum has been always reckoned as the first half-vertebra.

Of course it might be argued that in this particular instance centra 48 and 49 constitute the first two half vertebrae, and not centra 49 and 50 ; and similarly for all other cases.

Judging from analogous cases, such as that of Amia calva (Gadow 6, p. 202 and fig. 14), this latter view may possess rather more support. Until the question is settled by the investigation of the innervation of the myomeres in this region more cannot be said. The point of importance here is that a uniform system of nomenclature has been adopted for all the specimens examined. It may be mentioned that the mere size of the centra does not constitute an accurate indication of this point of junction. Though the first " halb-wirbel " centrum is usually smaller than that of the last " ganz-wirbel," yet a glance at fig. 2b will show that this is not always so. In this case centra 48 and 49 belong to the first half-vertebra in spite of the fact that centrum 49 is at least as large as any centrum of the whole vertebrae.

As the collector nerve lies along the outer side of the vena parietalis, injection of this with osmic acid rendered visible even the minutest branches which went to form it. The post-girdle nerves were also dissected with the help of the same reagent, which showed them up very readily in the fresh material used.

One hundred specimens in all were examined, of which fifty were males and fifty females. Of the males twenty-nine were adults ranging from 35 cm. to about 90 cm. in length, whilst forty of the females ranged between these sizes. The rest were old embryos about 25 cm. long, very nearly ready to be born, which for all intents and purposes, as will be seen below, could be reckoned as adults. The tabulated results of the points examined are given in Table I.