\ 
26 
NATURE 
| [Wov. 10, 188 
which will apply to structures present in the Lampreys, 
Sturgeons, Chimera,and Dipnoi, and will not apply to struc- 
tures present in Amphioxus. Gegenbaur’s conception of the 
inapplicability of the term Vertebrata to forms devoid of 
myotomes as are the Tunicata, is, it seems, more reason- 
able. But even this objection is removed by the fact 
that in the tail of some Ascidian larve, and in Appendi- 
cularia, there are indications of segmentation of the 
muscular tissue. 
However that may be, Mr. Balfour's account of the 
developmental phenomena exhibited by the various 
groups is of the greatest value, because it possesses 
three characteristics which also marked his first volume : 
it is complete as an epitome of all the very numerous and 
important contributions to the subject due to the Conti- 
nental and English embryologists who have written so 
abundantly of late years ; it contains a large amount of 
the author’s original unpublished observations ; and, lastly, 
it is no mere catalogue of the opinions of this and that 
authority, but is a critical treatise in which without arro- 
gance, but with argumentative skill, a definite view as to 
the significance of the phenomena described, even when 
these are obscure and difficult of interpretation, is put 
before the reader. This latter feature gives Mr. Balfour’s 
writings a special value, as pointing out lines of research 
for future observers. 
The chapter on the Elasmobranchii is chiefly based on 
the author’s original researches, which were published as 
a monograph. He has been extending his observations 
to the Cyclostoma and Ganoidei during the period in 
which he was also engaged in the preparation of the two 
volumes of the present treatise. Valuable original draw- 
ings (Figs. 38, 39, 40, 43, 45, 46 and 48) of sections of 
embryos of Petromyzon Planeri are given, and a cor- 
respondingly original account of the developmental 
history. Similarly the Ganoids, Accipenser, and Lepi- 
dosteus are illustrated by original drawings prepared from 
embryos supplied to Mr. Balfour by Prof. Salensky and 
Prof. Agassiz. In discussing the Amphibia much use 
has been made of the excellent figures given by Prof. 
Gotte in his great work on the development of Bomdéi- 
nator igneus, but here again Mr. Balfour is able to rely 
upon original observations upon the newt, carried out in 
his own laboratory by Messrs. Scott and Osborn. 
The fact that Mr. Balfour does not give us in a precise 
form a history of the development of the common frog 
from stage to stage, is explained by the special nature 
of his treatise, which aims at putting forward the gene- 
ralisations of embryology and dealing with the develop- 
mental phenomena of the whole range of animal forms 
rather than providing the commencing student with a few 
selected examples of growth from the egg. Much is said 
about the common frog in the chapter on Amphibia, and 
from the general statements which it contains, in addition 
to the statements definitely relating to the frog, a nearly | 
complete answer can be obtained to all questions which 
suggest themselves in relation to the main features of 
development in that animal. 
The Birds are treated next in order after the chapter 
on Amphibia, and necessarily the common fowl—on 
which Mr. Balfour published some years since, in con- 
junction with Dr. Foster, a separate work designed for 
the use of junior students—is the source from the study of 
which his facts are derived. In this chapter, and in that . 
on the Mammals, Mr. Balfour discusses the views of 
Kdélliker as to the origin of the mesoblast from the epi-— 
blast of the primitive streak, and other interesting points — 
raised, since his earlier work, by the observations of 
Braun on parrots and ducks, and of Gasser on geese. 
The chapter on Reptilia is remarkably short, owing to. 
the fact that very few observations have been made on 
members of this class, and that in many important points — 
they agree with birds. Original drawings relating to — 
Lacerta muralis and Chelone midas illustrate this section. 
In dealing with the Mammalia Mr. Balfour has to 
depend chiefly upon the recent researches of Ed. van 
Beneden and of Kdlliker, and his critical power and fair 
dealing is shown in the way in which he treats the points 
of disagreement between those two admirable investigators. 
The main outlines of our knowledge of the later develop- 
ment of the Mammalian embryo and its foetal appendages 
were laid down many years ago by Bischoff and by K6lli- 
ker ; but Mr. Balfour has given a particularly interesting 
account of the various modifications of the structure of 
the placenta presented by different mammals, illustrating 
his statement with woodcuts from the works of Prof. 
Huxley and Prof. Turner. With regard to the evolution 
of the placenta and the phyletic connection of the several 
forms seen in different recent Mammalia, he has some 
important original suggestions to offer. 
It is impossible to give any idea, in a review such as 
this, of the abundance of facts and the thoroughness of 
treatment to be met with in the portion of Mr. Balfour's ~ 
book which we have just noticed. It deals with the groups 
of Vertebrata one by one, and with the hundreds of ques- 
tions which the greater or less knowledge of the particular. 
group so far in the possession of embryologists, brings 
into existence in endless variety. The general results of 
such a method of exposition cannot be summarised ina 
review. 
But such summarising has been to a very considerable 
extent carried out by Mr. Balfour himself in the latter 
two-thirds of the present volume, which will be found the 
most readable, and in some respects the most important, 
part of the whole work. We have a chapter on the com- 
parison of the formation of the germinal layers and of 
the early stages in the development of Vertebrates, one on 
the ancestral form of the Chordata, and one treating of 
the mode of origin and homologies of the germinal layers 
in animals generally and of larval forms, their nature, 
origin, and affinities. To these chapters succeed twelve 
entitled “Organogeny,’’ which actually constitute a 
treatise on comparative anatomy, based upon embryo- 
logical data, under the headings (1) epidermis and de- 
rivatives ; (2) nervous system ; (3) organs of vision ; (4) 
auditory organs, olfactory organs, and sense organs of 
the lateral line; (5) the notochord, the vertebral column, 
the ribs, and the sternum ; (6) the skull ; (7) pectoral and 
pelvic girdlesand the skeleton of the limbs ; (8) the body 
| cavity, the vascular system, and the vascular glands; 
(9) the muscular system; (10) excretory organs; (11) 
generative organs and genital ducts ; (12) the alimentary 
canal and its appendages in the Chordata. 
In these chapters many of the facts which have been 
previously detailed in that part of the work devoted to the 
treatment of group after group are again brqught forward 
