NA TURE 



113 



THURSDAY, MAY 30, 1878 



BALFOUR ON ELASMOBRANCH FISHES 

 A Monograph on the Develop^nent of Elasmobranch 

 Fishes. By F. M. Balfour, M.A., Fellow and Lecturer 

 of Trinity College, Cambridge. (London : Macmillan 

 and Co., 1878.) 



MR. BALFOUR has finally completed and issued in 

 the form of an octavo volume the researches on 

 the embryology of the dog-fish and its allies, which he 

 commenced at the now celebrated zoological station of 

 Naples in 1874, His results have been made known from 

 time to time during the progress of his work by a prelimi- 

 nary paper in the Quarterly Journal of Microscopical 

 Science, October, 1 874, and by a series of articles in the 

 Journal of Anatomy and Physiology, the latter, indeed, 

 being identical with the successive chapters of the present 

 volume. Looking at the work as a whole, we may heartily 

 congratulate not only Mr. Balfour, but English science, 

 on the very great value of this contribution to knowledge. 

 Mr. Balfour, before entering upon the study of the deve- 

 opment of the shark-like fishes, had thoroughly qualified 

 himself for the task by a careful investigation of the de- 

 velopment of the common fowl, a subject which, although 

 it had always been and remains the favourite, be- 

 cause the most handy, for the embryologist's study, 

 yet yielded several new and interesting results to Mr. 

 Balfour's examination. The methods which are ap- 

 plicable to the hardening and slicing, staining and 

 clarifying of the embryo chick are precisely those which 

 it is necessary to employ in the investigation of the very 

 similar tg^ of the Elasmobranchs, and accordingly Mr. 

 Balfour had well trained himself [for the latter task. 

 The prominent position in Vertebrate morphology which 

 had been assigned to the group of Elasmobranch fishes, 

 through the researches of Gegenbaur, rendered a minute 

 examination of their developmental history urgent. It 

 had become clear that we have in these fishes the nearest 

 living representatives of the common ancestors of the 

 great group of Gnathostomous Craniate Vertebrata, and 

 it was to be expected that a full knowledge of their 

 ontogeny or individual development would carry us yet 

 further back in the line of "primitive Vertebrata, and yield 

 a mass of explanatory evidence, exhibiting the develop- 

 ment of complex and heterogeneous structures from 

 simpler and more homogeneous forms, likely to serve as a 

 satisfactory starting-point for all Vertebrate morphology. 



Mr. Balfour has shown in the course of his investiga- 

 tion of this subject not only that he is possessed of the 

 technical skill necessary for the manipulation of such 

 embryos, but that he is gifted with a very large measure 

 of patience and perseverance, and has, moreover, the 

 high critical and speculative capacities which the subject 

 demands for its full and successful treatment. 



We shall very briefly notice the successive chapters of 

 Mr. Balfour's monograph, and point to the more im- 

 portant novel observations [recorded, having especial 

 regard to those which may be considered as fundamental 

 for the morphology of Vertebrata, 



Mr. Balfour begins with the ovarian ovum of the Elasmo- 

 branch, this portion of his observations having been made 

 Vol. XVIII. — No. 448 



on the skate. He shows that the germinal vesicle atrophies 

 before impregnation. He then proceeds to describe the pro- 

 cess of segmentation, which, in its general features, is similar 

 to that of the bird, the only other ^%g containing so large a 

 proportion of food-material mixed up with the protoplasm 

 of the egg-cell . In the study of the division of the first- 

 formed cells resulting from the segregation and cleavage 

 of the mixed materials of the &gg, Mr. Balfour observed 

 and has figured the remarkable spindle-shaped condition 

 of the nuclei, which since has become such a prominent 

 subject of investigation through the initiative of Auerbach, 

 Strasburger, Biitschli, and van Beneden. Very remark- 

 able and important nuclei are also described and figured 

 as making their appearance in that part of the egg not 

 concerned in the process of cleavage or the formation of 

 the primitive disc of embryonic cells, and from their 

 occurrence Mr. Balfour is led to the conclusion that the 

 supposed distinction at this period of a purely embryonic 

 and a purely nutritive region in the ^gg of the Elasmo- 

 branch, is imaginary. This is important, because similar 

 observations have necessitated the abandonment of 

 similar erroneous divisions of the &gg of the fowl, of 

 osseous fish, and of cephalopods. The mass of cells 

 which form the smaU commencement of the embryo on 

 the surface of the great unsegmented yelk-mass divide 

 into an ectoderm and "lower layer-cells," and a true 

 " segmentation cavity " comparable to that of the frog is 

 described. The most important of Mr. Balfour's obser- 

 vations and suggestions which have a general bearing 

 upon the formation of the embryonic cell-layers through- 

 out the Animal Kingdom are those in which he points 

 out and gives its probable significance to the fact that 

 in the Elasmobranchs the primitive alimentary cavity 

 (archenteron) arises as a sort of in-pushing beneath 

 the hinder end of the embryo, a cavity being there formed 

 between the " lower-layer-cells " and the nucleated yolk. 

 The orifice of this cavity is spoken of by Mr. Balfour as 

 the " anus of Rusconi," and is identified by him accord- 

 ingly with the orifice so named in Amphibians. At the 

 same time it is tiot at this orifice that the final closure of 

 overgrowing ectoderm or epi blast takes place, that is 

 to say, of that layer of cells which, increasing by division, 

 spreads from the cleavage disc so as to gradually cover 

 in the whole of the large surface of uncleft yolk. The 

 gradually narrowing margin of these epibolic cells does 

 not in sharks have a centre coincident with the anus of 

 Rusconi ; in fact, the blastopore, as the orifice bounded by 

 this gradually narrowing margin is termed, lies behind the 

 embryo altogether. Mr. Balfour suggests (and it is neces- 

 sary to remember that his statements on this subject were 

 first published three years ago) that the primitive-streak 

 of the bird'' s blastoderm is a rudimentary representative 

 of this portion of the blastopore; it seems necessary to say 

 "this portion," and not the whole blastopore, as Mr. Bal- 

 four does ; for tracing these various structures back with 

 Mr. Balfour to the blastopore of the Amphioxus, we must 

 admit that in the meroblastic ova of Sharks and Birds the 

 blastopore has become greatly extended along the median 

 line and has its most anterior portion represented in the 

 anus of Rusconi of the Elasmobranch, a middle portion in 

 the orifice of final closure of the elasmobranch's blasto- 

 derm and the primitive streak of birds and mammals, 

 whilst a more posteriorly placed extension of the same 



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