NATURE 
5) 
THURSDAY, NOVEMBER 10, 1881 
BALFOUR'S “COMPARATIVE EMBRYOLOGY” 
A Treatise on Comparative Embryology. By Francis 
M. Balfour, LL.D., F.R.S., Fellow and Lecturer of 
Trinity College, Cambridge. Vol. II. (London: 
Macmillan and Co., 1881.) 
R. BALFOUR has brought out the second volume 
of his treatise with admirable punctuality, and 
- zoologists will find it no less valuable than the first. In- 
deed it is in many ways more attractive than the earlier 
volume, on account of the fact that the developmental 
history of the Vertebrata is here dealt with, and has an 
interest for a large class of anatomists who are not ad- 
dicted to the study of other organisms. Moreover, in 
treating of the Vertebrata (or Chordata, as he prefers to 
call them when the group is so extended as to comprise 
the Ascidians) Mr. Balfour has introduced a very con- 
siderable amount of original matter. 
The structure of the Vertebrata is not only more com- 
plex than that of other animals, but it is also better 
known, and has been more minutely discussed by ana- 
tomists ; and similarly the development of various Verte- 
brate types has been more keenly scrutinised than that of 
other forms. Amphioxus, the lamprey, the salmon, the 
dog-fish, the frog, toad, and newt, the turtle and lizard, the 
common fowl, the guinea-pig, rabbit, bat, and even man, 
have formed the subjects of numerous memoirs devoted 
to one or other of their phases of development. This has 
been going on for many years, in fact ever since Remak and 
KGlliker laid the foundations of what may be called “cel- 
lular embryology.” The chick, the frog, and the rabbit 
have during this period enjoyed the services of a class of 
workers difffering from those who have studied other ani- 
mals. The latter have been naturalists interested in the 
study of embryology as throwing light on the affinities and 
origins of animal forms; the former have been distinctively 
medical men, who have sought in the minute study of the 
origin of the tissues of man and other Vertebrate animals 
indications which may be of service towards attaining the 
great desideratum of modern medicine, viz. a thorough 
knowledge of the physiology (ze. the working of the me- 
chanism) of man. Accordingly, from an early period the 
methods of the histological laboratory have been applied 
to the study of the Vertebrate embryo, and that by a large 
number of accomplished investigators, whilst it is only 
quite recently that the naturalists, as distinct from the 
medical men, have learnt to apply the same methods to 
the study of all organisms. There is at the present moment 
a movement from both sides and a fusion of the hitherto 
separate streams of “ zoological” and “ medical” embryo- 
logy, which is marked as an epoch in the history of 
science by Mr. Balfour's treatise. 
_ The medical histologist and physiologist has learnt 
that if he would comprehend the process of the cleavage 
of the egg and formation of the blastoderm and pri- 
mitive organs he must not confine himself, as hitherto, 
to the limited area of comparison offered by the chick, 
the frog, and the rabbit; he must make common 
cause with the zoologist, and embrace the whole animal 
VoL. xxv.—-No. 628 
series in his view. He will, I cannot doubt, also soon 
openly acknowledge that the application of elaborate in- 
struments of measurement to the nerves and muscles of 
dogs, rabbits, and frogs has furnished what knowledge it 
can in reference to man, and that if physiology is to 
move out of a barren path the whole evolutional series 
connected with man—the lowest as well as the highest— 
must be made the subject of experiment. 
On the other side we find the field-naturalist—the lover 
of the forms and colours of animals—no longer content 
with a superficial study. To solve the problem which 
Mr. Darwin has succeeded in placing before him as the 
aim of his science, it is necessary that the minute struc- 
ture of all animals—their cellular anatomy and embryology 
—shall be as accurately known as is that of the rabbit 
and frog to physiologists. Accordingly it is becoming 
more and more usual to find naturalists trained in the 
histological methods originated by the medical physiolo- 
gists, and pursuing precisely the same inquiries as they 
do. 
Since the germ-layer theory was shown to apply not 
exclusively to the Vertebrata, but, in a modified form, to 
the whole animal kingdom, embryology has become one 
body of doctrine equally significant for the practical ends 
of the medical man and for the speculative conclusions of 
the philosopher and naturalist. This fact is abundantly 
evident from Mr. Balfour’s two volumes; in the earlier 
as in the present the chief aim is to trace the history of 
the units of structure known as cells from the parent egg- 
cell until the adult form is attained. The doctrine of 
cell-structure and that of evolution taken together serve 
to unite the interests of scattered and sometimes reci- 
procally contemptuous groups of scientific men—the 
physiologist and the naturalist will turn each with equal 
pleasure and profit to Mr. Balfour's treatise. 
The embryology of the Chordata is first of all treated 
of, in the present volume, in zoological order. The 
terms Cephalochorda (for Amphioxus), Urochorda (for 
the Tunicata), and Craniata, which were proposed as 
divisions of Vertebrata in my “ Notes on Embryology 
and Classification,” are used, with some modification, by 
Mr. Balfour. Instead of Craniata the term Vertebrata is 
used, whilst in place of Vertebrata as formerly applied, 
the term Chordata is used. This change is open to 
objection, chiefly on the ground that it is more convenient 
to retain so well-known a term as Vertebrata for the more 
important group, and not to sink it in subordination to an 
unfamiliar term: also, as it seems to me, on the ground 
that the implication in both words “Chordata” and 
“Vertebrata,” as used by Mr. Balfour, is delusive. All 
animals with a “ chorda” would not necessarily take their 
place in the group of pharyngo-branchiate Chordata pos- 
sessed of a tubular nervous axis and myelonic eyes, to 
which rather than Chordata the old name “ Vertebrata’ 
is appropriate—the Tunicates having been assimilated 
by the old-established group in the course of a natural 
process of the growth of knowledge. 
The defence of the limitation of the term Vertebrata to 
the Craniate Vertebrates on the ground that they alone 
possess “ vertebrae,’ raises the whole question of what 
we are to understand in the widest sense by the words 
“vertebrae” and “vertebrate.’’ It seems to me to be 
| difficult to construct a definition of either of these words 
Cc 
