THURSDAY, JANUARY 14, 1904. 
EXPERIMENTAL STUDIES IN DEVELOPMENT. 
Einfiithrung in die Experimentelle Entwickelungs- 
geschichte. By Prof. Otto Maas. Pp. xvi+203. 
(Wiesbaden: J. F. Bergmann, 1903.) Price 7 
marks. 
NE of the most fascinating branches of biological 
inquiry is that concerned with the investigation 
of those factors that underlie organisation and deter- 
mine the course of development of the individual from 
the egg to its adult condition. From old time the 
question as to why a hen’s egg should give rise to a 
fowl and not to a lizard or a mammal is one that has 
invited but never been met with a satisfactory answer, 
any more than the agencies have been recognised that 
direct and determine the orderly series of cell divisions 
culminating in the production of a specific form with 
all its marvellous organs and complex tissues. 
It was formerly believed that, in some mysterious 
fashion, the actual structure of the adult lay concealed 
in the egg, much as the flowers of some of our trees 
can be detected in a resting condition, while it is still 
winter, by stripping off the bud scales that enfold them. 
When this view had been shown to be both logically 
and as a matter of experience untenable, the doctrine 
of epigenesis displaced it, but this, too, failed to 
provide a satisfactory basis on which a comprehensive 
explanation of the phenomena could be built up. Thus 
in quite recent times a revival of the evolution-theory 
has arisen, not, indeed, in the older and cruder form, 
but as promulgated by Weismann and his followers 
it has appeared to throw light on, and indicate a reason 
for, the remarkable phases passed through by the cell- 
nucleus during its division, and at the same time it 
took cognisance of the extraordinary phenomena that 
precede and accompany sexual reproduction. It has, 
however, been subjected to strenuous criticism, and 
weighed in the balance it, like its predecessors, has 
been found wanting. 
The centre of gravity of current investigation is 
shifting again from the nucleus to the extra nuclear 
cell-protoplasm (cytoplasm). As the result of ex- 
periment, it has become certain that this part of the 
cell has to be reckoned with in any theories that pre- 
tend to group the facts together, and it is pretty 
certainly a good deal more than a mere nutritive sub- 
stance which simply furnishes the nucleus with sub- 
stances that may enable the latent possibilities of the 
latter to be converted into actual entities having the 
specific quality of form and other properties. The old 
definition of the cell as a mass of protoplasm contain- 
ing a nucleus is still found to hold good, but the parts 
played by the two constituents admit of more precise 
delimitation than was the case even a few years ago. 
A step of no small importance was made when it 
was discovered that by centrifugalising fertilised 
frogs’ eggs, so as to drive the yolk up to one end of 
the egg, the course of segmentation becomes artificially 
meroblastic. Thus a condition is produced which is 
NO. 1785, VOL. 69] 
NATURE 241 
actually met with in many eggs (e.g. of molluscs) im 
which the yolk is present in large quantity and is- 
unequally distributed. 
Still more important was the further discovery that 
the first few blastomeres of a fertilised segmenting egg 
could be separated and induced to continue their de- 
velopment as isolated individuals. For this afforded. 
an opportunity of deciding whether the organism was 
the product of its cells, or the cells of the organism. 
The results strongly point in the latter direction. The 
same conclusion is reached from the experiments of 
Hertwig, who by compression succeeded in causing 
the early cells of the embryo to take up abnormal 
positions, but the organisation of the larva did not 
then follow the cell-arrangement, but superseded it. 
The experiments with isolated blastomeres do not give 
the same results in all cases. Thus, if they are 
isolated at the first segmentation of the ovum of an 
Amphioxus, each gives rise to a small but perfect 
embryo, and thus behaves as though it were a small 
egg. In the sea-urchins, the isolated cells at first 
continue to develop as thcugh the missing part were 
stili present, that is, they give rise to partial embryos. 
But very soon the form of the normal embryo at the 
corresponding stage is made good, and small but 
perfect larvae may result. Yet another example is seen 
in Beroé and some other animals, in which, whilst 
segmentation at first goes on as though the isolated 
part were a small egg, at later stages the embryo ex~ 
hibits various structural defects. 
It is very important to notice that these various types 
of behaviour do not depend on the nuclei. It might 
be thought that as the small larve, at first often de— 
fective (sea-urchins), had arisen from cells the nuclei 
of which had arisen by division from that of the 
original ovum, the defective character should be corre- 
lated directly with this fact. The nuclei might be 
supposed to have diverged in character, so that, for 
example, that of one cell contained in itself the latent 
potentialities of a definite half or other portion of the 
embryo. But such an explanation is directly con- 
tradicted by the facts as shown in Amphioxus, and 
would not be easily reconciled either with such cases 
as that of Beroé or by the commonly occurring modifi— 
cation of the further processes whereby small, but 
otherwise perfect, larvae may arise in spite of the 
initially different mode of segmentation. Moreover, 
it has been shown that when freshly fertilised eggs are- 
shaken so as to separate off portions of the cytoplasm 
before segmentation, modifications are produced very 
similar to those that occur in separated blastomeres. 
This appears to tell conclusively in favour of the great 
importance of the cytoplasm as a factor in determining 
the progress of development. In fact the egg, as has 
been well said, is itself an organism. Not that the 
parts characteristic of the adult are there present in 
esse, but the substance, the primordial materials out 
of which the early structures are severally built up, 
is actually present in the unsegmented egg. 
There is some direct evidence available on this point. 
In many eggs differences of colour or texture can be 
seen to occupy definite positions in the egg, and if it 
55) 
is rotated these zones often rearrange themselves,. 
M 
