NA TURE 



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THURSDAY, NOVEMBER 20, 1919. 



VERTEBRATE EMBRYOLOGY. 



Text-hook of Embryology. Vol. xi. Vertebrata 

 ■with the Exception of Mammalia. By Prof. 

 J. Graham Kerr. Pp. xii + 591. (London: 

 Macmillan and Co., Ltd., 1919.) Price 31^. 6d. 

 net. 



THE second volume of this important text-book 

 of embryology deals with all the vertebrate 

 groups with the exception of the mammalia. It 

 is unnecessary to say that a more highly qualified 

 authority than Prof. Graham Kerr to expound the 

 many intricate problems of the subject could not 

 be found, but what does impress the reader is the 

 clear insight into the depths of these problems 

 that the author has gained by his own detailed 

 and important original investigations. 



It is not surprising to find that, in this compre- 

 hensive treatise, the archaic but unfortunately 

 inaccessible mud-fish — Lepidosiren — plays a pre- 

 dominant role, and that our old friends of the 

 embryological laboratory — the chick and the tad- 

 pole — are relegated to minor parts ; but Prof. 

 Kerr convinces us that this is as it should be. 



The highly specialised vertebrates that we can 

 get in abundance for our class work are not the 

 best types in which to search for the clues to the 

 solution of the many problems of vertebrate em- 

 bryology. It is rather in the more archaic forms 

 with larger histological elements and a primitive 

 anatomical structure — such as the Dipnoi, the 

 Ganoidei, and the Urodela — that the embryologist 

 has found by experience that he obtains his most 

 satisfactory results. 



The need for a text-book of this description 

 which deals fully with the fundamental problems 

 of embryology, as distinct from a text-book that 

 deals only with the development of a few selected 

 types, has long been felt, and a brief reference 

 to one or two of these problems may be given to 

 indicate the manner in which the author expounds 

 them in the light of modern embryological 

 research. 



There has been no more highly controversial 

 problem in embryology than the question of the 

 origin of the nerves from the central nervous 

 system. Prof. Kerr describes in some detail the 

 result of his own researches on the development of 

 the motor nerves of Lepidosiren, and gives a 

 critical summary of the theories of His, Balfour, 

 and Hensen. His own opinion is finally expressed 

 and may be very briefly indicated by the following 

 sentence (p. iii): "It is suggested that the 

 development of the actual nerve fibril is simply 

 the gradual coming into view of a pathway pro- 

 duced by the repeated passage of nerve impulses 

 over a given route." 



The origin of the paired appendages of verte- 

 brata is another of these debated problems on 

 which distinguished morphologists have held very 

 divergent views. Here, again, the author sets 

 NO. 2612, VOL. 104] 



before the student the " branchial theory " asso- 

 ciated with the name of Gegenbaur, and the con- 

 tinuous "lateral fin " theory which was supported 

 by Balfour, before expounding his own views in 

 what he calls the "external gill hypothesis." 

 This hypothesis is based on the supposition that 

 the external gills extended further back than they 

 do in any living vertebrate, and that, being 

 potential organs of support, and also potential 

 organs of movement, as indicated by their flicking 

 movements in some existent larva, they became 

 transformed into purely locomotive and support- 

 ing paired appendages. The limb girdle on this 

 hypothesis is a modified branchial arch skeleton, 

 shifted backwards as in the theory of Gegenbaur. 



With the lateral fin theory already so well estab- 

 lished in this country, it is not likely that Prof. 

 Kerr will find his views in this matter generally 

 accepted, but there is so much that is interesting 

 and ingenious in the way in which his hypothesis 

 is expounded that the student must benefit by its 

 careful consideration. There are many other 

 problems of absorbing interest, to which space 

 does not permit us to refer, discussed in the spirit 

 of just consideration of the views of previous 

 writers, and a clear expression of the author's 

 own opinion. This is the feature which com- 

 mends the volume most strongly to the student 

 who is capable of appreciating something more 

 than a plain statement of the facts that have been 

 discovered. 



But a word of high praise must also be 

 given for those parts of the work that are purely 

 descriptive. It is always a difficult matter to 

 condense into the allotted space the main results 

 of exhaustive researches, but Prof. Kerr has 

 accomphshed this part of the task with great 

 skill and judgment. In some cases, perhaps, a 

 little more expansion would have been advisable. 

 For example, in the chapter on the development 

 of the brain a fuller explanation, with a figure, 

 of what is meant by the term " .\rchipallium " 

 would be most useful, or, again, in the description 

 of the development of the vertebral column of 

 Sphenodon, in which the student, puzzled by the 

 myotomes being opposite to the protovertebrae in 

 Fig. 152 C, but alternate with them in Fig. 152 B, 

 will find no key to the puzzle in the text. But 

 minor criticisms such as these seem out of place 

 in reviewing a book which has so many merits. 



We are glad to find that in writing this text- 

 book Prof. Kerr has not withheld from us the 

 fruits of his ripe experience as a teacher and in- 

 vestigator, for we find in chap. x. a most excel- 

 lent general account of the development of the 

 chick, illustrated by many good figures, and 

 accompanied by practical instruction in laboratory 

 methods. This chapter will prove to be of great 

 value to the beginner and to his teachers. More- 

 over, in the last two chapters we are given most 

 interesting and useful comments on the practical 

 study of the embryology of the lower vertebrata 

 and on the guiding principles of embryological 

 research. Prof. Kerr has rendered a great service 

 I to scientific students by the publication of this 



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