114 



NATURE 



{May 30, 1878 



structure (blastopore) is seen in the actual orifice of final 

 closure of the bird' s blastoderm at the antembryonal pole 

 of the yelk-sac. The continuity of the nervous and 

 alimentary tubes, after closure of the Rusconian anus, is 

 a striking feature which Mr. Balfour shows to be common 

 to Elasmobranchs, Ganoids, Osseous fishes, Amphibians, 

 Amphioxus, and Ascidians. To Kowalewsky we are 

 indebted for the first observation of this remarkable dis- 

 position in various types of lower Vertebrata, and its full 

 significance is not yet understood. 



The next point of great importance which we find in 

 Mr. Balfour's monograph is the derivation of the noto- 

 chord from the hypoblast or archenteron, from which also 

 the protovertebrae are developed constituting the meso- 

 blast. That the vertebrates' body-cavity, like that of 

 other animals, was primitively a portion of the alimen- 

 tary cavity appears likely from this observation, coupled 

 •with Kowalewsky' s more recent results as to the develop- 

 ment of Amphioxus, whilst it also seems likely that the 

 notochord made its first appearance as an organ apper- 

 taining to the alimentary tract, from which it became 

 gradually separated in function and in structure. 



The next thing which we come to is of even more 

 special interest for the limited department of Vertebrate 

 morphology. The unpaired and the paired fins alike 

 make their first appearance in the Elasmobranchs as 

 lateral ridges of epiblast, and Mr. Balfour accepts the 

 hypothesis that the limbs are remnants of continuous 

 lateral fins. The muscles of the limbs are shown to be 

 derived from the "muscle-plates" of the body which 

 develop from protovertebrae. 



It is to the nervous system that some of Mr. Balfour's 

 most original and important observations have reference. 

 He has elsewhere conclusively shown that, contrary to 

 Stieda's statements and in accordance with Owsjan- 

 nikow' s, the spinal nerves of Amphioxus have no anterior 

 •roots, that is have only dorsal roots. He now shows that 

 the early condition of the spinal nerres of Sharks agrees 

 with this, they having at first no anterior roots. An 

 enigmatical commissure parallel to the medulla unites the 

 posterior roots in the embryo. The cranial nerves — exclu- 

 sive of the first and second, and the nerves to the orbital 

 muscles which have peculiar features of their own — are 

 showntoretain permanently the primitive condition implied 

 in the absence of anterior roots. The vagus nerve is shown 

 to be the result of the morphological fusion or concrescence 

 of several segmental nerves — their separate roots (which 

 are all dorsal ones) being " caught " (so to speak) in the 

 sharks in process of disappearance. The identity of the 

 nature of these roots with those of the following spinal 

 nerves is shown by the connection with them of the 

 enigmatical commissure above mentioned. 



The segments which are represented in the Vertebrate 

 liead have been reduced and blurred by the integration of 

 that region of the axis, but by the aid of the embryonic 

 relations of the cranial nerves, and of a very important 

 and remarkable series of cavities representing the body- 

 cavity of the head (the terms are not contradictory since 

 "head" is chiefly developed from "body") in a seg- 

 mented condition, which Mr. Balfour has discovered in 

 the Elasmobranchs, he is able to indicate distinctly at 

 least seven post-oral segments in the cephalic axis, and 

 he adduces cogent reasons for supposing that a larger 



number existed, and have been suppressed by a kind of 

 integration. 



As to the brain, Mr. Balfour gives important evidence 

 against the fanciful interpretations of Miklucho-Maclay, 

 whom, strangely enough, Gegenbaur has followed. What 

 most persons call mid-brain, Miklucho-Maclay has identi- 

 fied with the thalamencephalon or twixt-brain {Zwischen- 

 him) of other Vertebrates, being induced by the large 

 size of what is usually called the Elasmobranch cere- 

 bellum to consider it as the mid-brain. Mr. Balfour 

 gives strong embryological evidence against this view. 



As to the relation of nerves to the primitive germ 

 layers, it is shown (in accordance with Hensen's observa- 

 tions in Mammalia) that the spinal nerves are outgrowths 

 of the medulla, and Mr. Balfour, though he is unable 

 actually to demonstrate it, yet brings a variety of evidence 

 to show that the whole growth of the nerves is a centri- 

 fugal one, and that therefore the peripheral elements of 

 the nervous system may have the same"primary origin as 

 have the central. 



The important question as to how the axial medulla 

 arose, and whether it is homogeneous with the ventral 

 nerve-cord of Annelids and Insects is discussed in the 

 light of the facts ascertained as to the development of 

 the nerve-medulla in dog-fish. Mr. Balfour on the whole 

 favours the view that the nervous system of elongated 

 animals consisted primitively of two lateral cords, and 

 that in Annelids and Insects these cords have met and 

 fused below the alimentary tract, whilst in Vertebrates 

 they have met and fused above the alimentary tract. 



A curious modification of a part of the nervous system, 

 the meaning of which is as yet entirely beyond the most 

 hazardous speculation of either physiologist or morpho- 

 logist, is shown by Mr. Balfour to present itself in the 

 supra-renal bodies. They develop from ganglia of the 

 sympathetic portion of the nervous system. 



Lastly, we have to mention the series of results relating 

 to the origin of the renal organs and the ducts of the 

 generative system. These are already the most widely 

 known and discussed, though possibly not actually the 

 most important of Mr. Balfour's numerous discoveries. 

 The fact that Prof. Semper, of Wiirzburg, occupied him- 

 self with the investigation of the renal organs of Elasmo- 

 branchs at the same time as did Mr. Balfour, and that 

 the two investigators nearly simultaneously arrived 

 at the same results, has given a special value to 

 this part of the observations embodied in the present 

 monograph. Mr. Balfour shows that the Vertebrate 

 kidney is a condensation of tubules, of which primitively 

 one pair existed in each segment of the body, opening 

 into the body-cavity each by a ciliated funnel, and therefore 

 exactly comparable to the segmental-organs or nephridia 

 of the Annelids. Whether, as Gegenbaur holds, these 

 organs were originally a simple pair which became 

 segmented, that is, provided with a separate funnel in 

 each metamere or body-segment, or whether each tubule 

 or nephridium originally opened to the exterior, so that 

 an unconnected series of nephridia existed on each side 

 of the body — a pair in each segment — which subsequently 

 became joined to one another by longitudinal common 

 ducts— one on each side of the body — is still matter for 

 consideration. 



The adaptation of the most anterior funnel to the 



