SCIENCE. 
97 
SATURDAY, AUGUST 28, 1880. 
BOOKS RECEIVED. 
A TREATfSE 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 oi 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 phylogenetic 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 
wheredt is evenly distributed ; (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 and interesting. 
In summary of the group Arthropoda, the genealogy of 
the Tracheataand 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. 
