SCIENCE. 



97 



SATURDAY, AUGUST 28, 1880. 



BOOKS RECEIVED. 



A Treatise on Comparative Embryology. By 

 Francis M. Balfour, M. A., F. R. S., in two volumes. 

 Vol. 1. Macmillan & Co., London. 1880. 



Macmillan & Co. have recently forwarded to us this 

 very interesting volume of over 300 pages, abundantly 

 illustrated, the first part of a work upon which we 

 believe the author has been constantly occupied since 

 the publication of his " Development of the Elasmo- 

 branch Fishes," in 1878. The second volume is still 

 in press, and will deal with the Embryology of the Ver- 

 tebrata ; while this, the first volume, is devoted to the 

 Invertebrata, omitting the Protozoan forms, and it in- 

 cludes in the beginning of the book an outlined history 

 of the Ovum, as it appears in both the Vertebrate and 

 Invertebrate types. 



This is by far the most important book that Mr. Bal- 

 four has undertaken, and, in practical importance, takes 

 the precedence of any work that has yet appeared in 

 this branch of biological science. 



In his "Elements of Embryology," written with Prof. 

 Foster of Cambridge in 1874, and his " Elasmobranch 

 Fishes," besides numerous contributions to the Quarterly 

 Journal of Microscopical Science, and the societies, the 

 author has won a position among European embryolo- 

 gists which makes this work doubly valuable. Merely 

 for an expression of his opinion on many mooted ques- 

 tions, the book will be welcomed on both sides of the 

 Atlantic ; but it contains, moreover, as full a history of 

 every form as scientific investigation up to the present 

 time has furnished, with a manifest endeavor throughout 

 to do justice to every investigator. The absence of such 

 a work has long been felt by American students. Few 

 of our libraries have been able to obtain, to anything like 

 completeness, the works of biological specialists ; in 

 great measure because such works appear in the form 

 of scattered memoirs, difficult to procure even at the 

 time of publication. In the full field of French, German, 

 Italian and Russian investigations, the danger of com- 

 pletely overlooking the researches of others is constantly 

 discouraging. It may be in appreciation of this general 

 want that the author has placed full notices of his 

 sources of information at the end of each chapter 

 where such reference is made, and, in addition to an 

 index of subjects, has inserted at the close of the volume 

 a classified bibliographical index. This renders each 

 subject exceptionally clear, and places the student in a 

 much fairer way of hunting up the literature of his 

 specialty than has been possible hitherto. In these 

 respects, the book is a model for works of this character. 



The science of embryology, now ripe for an eclectic 

 work of this description, has grown rapidly from its 

 infancy in the middle of the present century to the 

 importance of a separate and elaborate branch. With 

 its voluminous literature, it is strange that with one 

 exception, a small volume by Packard, no attempt has 

 been made to collate opinions or handle the subject as a 

 whole. In the phylogeneMc light alone, Embryology 

 ranks as a vital portion of Biology ; in this connec- 

 tion may be quoted a few lines from the introduction : 

 " It has long been recognized that the larvae and 

 embryos of each group pass, in the course of their 

 development, through a series of stages in which they 

 more or les completely resemble the lower forms of the 

 group." The author shows the bearing of the Darwinian 

 theory upon this fact. While morphology may establish the 

 relations of genera, we turn to Embryology for the basis 

 of a wider classification. Its bearing upon Comparative 

 Anatomy is a patent fact. So it is in the interest of the 

 history of development, or in the relation of a given type 

 to its progenitors, as well as in the morphology and 



physiology of individuals that embryology is of constantly 

 increasing importance. This is, in part, pointed out by the 

 author in the introduction of the work. More specifically 

 he states the aims of the present work as two-fold : (1) To 

 form a basis for Phylogeny (or the history of the race or 

 group) ; and (2) to form a basis for Organogeny (or the 

 origin and evolution of organs). 



In course of a review of the phenomena of reproduc- 

 tion, as witnessed among the Protozoa and Metazoa, the 

 transition from single to compound organisms is clearly 

 stated : " It must be remembered that a single individual 

 Metazoon, is equivalent to a number of Protozoa 

 coalesced to form a single organism in a higher state of 

 aggregation. It results from this that the segmentation 

 of the ovum which follows the sexual act may be com- 

 pared to the product of conjugation breaking up into 

 spores, the difference between the two processes consist- 

 ing in the fact that in the one case the spores separate 

 to form an independent organism, while in the other they 

 remain united, and give rise to a single compound 

 organism." 



The ovum is treated of in the first chapter as the 

 natural point of departure in the cycle of development — 

 first in its general, then in its special histories in different 

 types. In the second chapter upon Impregnation and 

 Maturation, an account is given of the remarkable 

 researches of Fol and Hertwig, which surpass in the 

 minute history of these changes the observations of any 

 other naturalists. A chapter on Segmentation closes 

 this introductory portion of the work. In view of the 

 fact that this phenomenon hinges upon the disposition of, 

 the presence, or the absence of food-yolk, the author 

 proposes terms for three corresponding types of ova, as 

 follows : (1) Alecithal for ova without food-yolk, or 

 where'it is evenly distributed ; 1^2) Telolecithal, where the 

 yolk is concentrated at one pole ; (3) Centrolecithal, 

 where the yolk is concentrated in the centre. 



The reader is now ready for Part I, Systematic Em- 

 bryology, in which the history of each group is treated 

 from the formation of the germinal layers onwards, 

 beginning with the simple parasitic forms, the Dicyemidae 

 and Orthonectidae, passing through each invertebrate 

 family whose development has been studied, and closing 

 with the Echinodermata and Eteropneusta. 



A detailed review, even of the author's conclusions, 

 would be obviously out of place. Attention may, how- 

 ever, be called to one or two passages of interest, not 

 only to the specialist, but to the general student of 

 biology. The Coelenterata form an attractive group from 

 the tact that they rarely, if ever, pass from the two 

 layered condition, and the lowest forms, even when 

 adult, "do not rise in complexity much beyond a typical 

 gastrula." The larval form, the planula, is common to 

 all except the Ctenophora. Referring to this, the author 

 remarks: "Paradoxical as it may seem, it appears to 

 me not impossible that the Coelenterata may have had an 

 ancestor in which a digestive tract was physiologically 

 replaced by a solid mass of amoeboid cells." 



The chapter on the development of the Mollusca is 

 very full £>nd interesting. 



In summary of the group Arthropoda, the genealogy of 

 the Tracheata and Crustacea tends to throw doubts upon 

 the uniting of the whole of the arthropoda into one 

 phylum. In the first place, the Tracheata are descended 

 from some terrestrial annelidan type allied to Peripatus. 

 [This is the interesting proto-tracheate form collected by 

 Mr. Moseley on the Challenger expedition, and found by 

 him to possess trachea and nephridia, two organs which 

 respectively demonstrate its affinities in opposite lines to 

 the tracheate and annelidan groups.] The Crustacea on 

 the other hand are clearly developed from a phyllopod- 

 hke ancestor, which can in no way be related to Peri- 

 patus. The conclusion that the Crustacea and Tracheata 

 belong to two distinct phyla, is moreover confirmed by 

 their development. 



