170 



NA TURE 



[Decemhek 22, 1S9S 



the bony skeleton exclusively from the connective tissue, 

 and we no longer imagine that these tissues arose in situ, 

 owing to changes which, as their causes cannot be 

 explained, are by themselves not understandable. 



In connection with the above consideration may be 

 mentioned thefoUowing important views(p. 152): Although 

 treated as an hypothesis, it appears likely that enamel, 

 dentine, bone substance, are all of ectodermal origin, and 

 the same is probably the case with cartilage. And p. 

 200 : There are reasons for assuming, although not yet 

 absolutely sure, that the whole mass of the placoid scale, 

 including the bone, arises from the ectoderm. Anyhow, 

 all the hard structures, and all their bony formations are 

 derived froni the integument. In the higher creatures the 

 bone-forming material is already mixed up with the 

 mesoderm, and our author thinks that the perichondrium 

 has received its bone-forming elements likewise from the 

 integument. ' Some skleroblasts are certainly identical 

 with osteoblasts, and these latter become eventually bone- 

 corpuscles. 



Anyhow, at last, the bugbear of the fundamental 

 difference between tegumentary (so-called secondary, in 

 reality primary) and chondral bone, and that between 

 endo- and ecto-chondral bone, is recanted. Gegenbaur 

 himself had introduced these differences, which have 

 been used for the last thirty years as an all-powerful 

 conjuring formula. He has laid the ghost originally 

 raised by himself ; laid perhaps too effectively, as the 

 old terms could easily be made to receive a corrected 

 meaning, in harmony with the old well-meant distinction, 

 if — instead of primary and secondary bones in the old 

 sense — we put primary and secondary elements of the 

 primordial skeleton. 



The author (p. 186) is in favour of the connection of the 

 so-called notochord (which he would rather not straight- 

 way call a notochord) of Cephalodiscus, Rhabdopleura and 

 Enteropneusta, with that of the Tunicates. The lucid 

 treatment of this important structure may be used as 

 an example of how the conditions prevailing among 

 Invertebrata can be made to throw light upon the more 

 complicated V'ertebrata. He argues, namely, as follows : 



" The origin of the chorda from entoderm, from the gut- 

 wall, is phylogenetically not understandable. Such a 

 string of cells must originally have had another function 

 than that of support. It is therefore reasonable to 

 derive the chorda from some gut-diverticulum, such as 

 actually does exist at the very place whence phylo- 

 genetically the chorda must have begun ; the diverti- 

 culum, as well as the chorda arising, moreover, at the spot 

 where ento- and ecto-derm meet. And although such 

 diverticula no longer exist in Tunicates, their ancestors 

 must necessarily have possessed them, because some of 

 the recent Tunicates have a chorda. Moreover, the 

 occurrence of such diverticula in several otherwise 

 divergent forms, as, for instance, Cephalodiscus and 

 Rhabdopleura, indicate a former, more widespread 

 existence." 



All this is very satisfactory, but how does it fit in with 

 what we read further on about the brain and the spinal 

 cord? 



1 It is difficult to see how this tegumentary origin of cartilage (for vvliicti 

 also see p. 152) can bereconciled with the statement made on p. 241—" .'\n 

 additional argument against the exclusive origin of cartilage from meso- 

 derm is the transformation of chordal into cxrtil.ige cells "—even if this so- 

 called chorda] cartilage had not been shown hy Zykoff and others to be an 

 isly inlcrprclcd observation. 



NO. I 52 I, VOL. 59] 



Gegenbaur, namely, has always held that the brain is the 

 older, and the spinal cord the newer formation. Con- 

 sidering the importance of this question for the origin of 

 the Vertebrata from Invertebrata, we turn eagerly to what 

 he has now to say on this point. But instead of receiving 

 comfort, we are led into a maze. See for this the follow- 

 ing three statements : 



(P. 724.) The phylogenetic value of the ontogenetic 

 mode of origin of the spinal cord, namely, as a further 

 continuation, or budding, from the brain, can be con- 

 tested. Because such a mode of formation would pre- 

 suppose a condition in which the spinal cord had 

 preserved its ectodermal position in the ancestors of the 

 Acrania. Only the epichordal nerve-string of the Tuni- 

 cates represents a sort of early stage of the spinal cord, 

 but in reality it is not yet a spinal cord. 



(V. 725.) E.xclusively ontogenetic treatment of this 

 question shows that the spinal cord is phyleticalK 

 produced by a successive budding from the archen- 

 cephalon. But if this were really so, then the otherwise 

 so well-founded connection with the Tunicates would 

 disappear. . . . 



On p. 779 the author holds that the mode of formation 

 of the cord in Petromyzon and in Teleostei, as a solid 

 string with subsequent appearance of a central canal, is 

 more primitive than the early formation of a semi-canal 

 of the medullary plate as observed in all the other 

 Vertebrata. 



These three statements are not easily reconciled, and 

 that ontogeny supports the budding mode is an asser- 

 tion at least surprising. But behold, on p. 718 and p. 719, 

 concerning Tunicates, we are told that : 



" Die Medullarplatte senkt sich in die Tiefe, besonders 

 hinten, wodurch eine Taschenform entsteht. Die nach 

 vorn sich weit oeffnende, hinten geschlossene Tasche 

 erstreckt sich immer weiter nach hinten, indess ihre 

 aeussere Oeffnung sich verengt, und als Neuroporus 

 weiter bestcht." 



At first sight the reader will think that nui/i hinten 

 is a lapsus calami, instead of naili vorn, but this 

 is not the case. The sentence also reads as if neuropore 

 and blastopore were the same, but a few lines further on 

 we are categorically informed that certain other features, 

 for instance the neurenteric canal, "are cenogenetic, 

 and have to be passed over." In fact the author 

 translates into growth backwards what ontogeny clearly 

 shows to be the opposite, namely, the closing in of the 

 canal from the blastopore forwards. Consequently the 

 secondary nature of the spinal cord, as a budding from 

 the brain, receives no support from ontogeny, while 

 according to the author himself it can be contested upon 

 phylogenetic grounds ! 



For the rest we cannot do more than single out a fe>«i> 

 paragraphs as samples, be they typical illustrations of the ' 

 masterly treatment of the whole work, or be they fallings' 

 short of, or running contrary to, our expectations. 



The author emphasises the great difference betweea, 

 Sauropsida and Mammalia, and as we read further 

 through the book all the differences are made much of^ 

 until the only group worthy of possible ancestral relation- 

 ship arc the .Amphibia, notably .Anura. This old view- 

 has been revived, and it is difficult to say if this unfor- 

 tunate notion of Amphibian descent " mit Umgchung der. 



