OcrToBER 12, 1916] 
NALURE 
12! 
last twenty years by selective mating, but the study of 
the laws governing the development of the germ into 
the adult organism—in a word, of, experimental em- 
bryology—might eventually throw a great deal of light 
on the laws of heredity. 
After alluding to the work of His, who sketched. out 
the programme of the new science, Prof. MacBride 
described the work of the first experimenters—Roux, 
Hertwig, and Driesch—in some detail. He pointed 
out that the results obtained by these zoologists led 
them to conclusions about the nature of development 
which were fundamentally opposed to one another ; for 
Roux, having produced half-embryos by destroying 
one blastomere of the two-cell stage of the frog’s 
egg, supported the principle of “‘ specific organ-form- 
ing regions of the germ,” whilst Driesch, having 
reared a perfect Echinoderm larva of diminished size 
from one of the first four blastomeres of an Echinus 
egg, asserted that ‘‘the fate of a cell was a function 
of its position in the embryo,” and in this conclusion 
he was supported by Hertwig, who attempted to in- 
terpret Roux’s results in a different manner. Even 
Roux admitted that although half-embryos were 
formed at first, if they survived they regenerated the 
missing parts; Roux accounted for his results by 
supposing that each region of the germ had its 
peculiar organ-forming substance, or ‘‘idioplasson,”’ 
which conferred on it the power to develop into 
a definite organ; the regeneration of lost parts he 
attributed to a special substance, which he called 
‘reserve-idioplasson,’’ which came into play only 
when mutilation had occurred. Driesch assumed, on 
the other hand, the existence of a purposeful “ ente- 
lechy,”’ or *‘ psychoid,’’ inhabiting the living material, 
and even when, as in his experiments with Ctenophore 
eggs, he found that isolated blastomeres gave rise to 
partial larvae, he did not conclude that detinite organ- 
forming substances were localised in each of the first 
eight ‘blastomeres; but rather that in these eggs the 
cytoplasm was so specialised or ‘‘stiffened’’ that the 
indwelling entelechy could not mould it to its will. 
The definite proof of the existence of organ-forming 
substances—a proof which was regarded as one of the 
great advances made by experimental embryology— 
was brought by Crampton and by Wilson in their 
studies of the developing eggs of Mollusca. In the 
developing egg of Dentalium and of some other Mol- 
lusea the first cleavage appeared to divide the egg into 
three cells, but one of these cells was a mere pro- 
trusion devoid of a nucleus, termed the first polar lobe, 
which was reabsorbed before the next cleavage. At 
the next cleavage five cells were apparently produced, 
but again one of these was a transitory second polar 
lobe, which melted into one of the four blastomeres 
before the cleavage to form eight cells began. If the 
first polar lobe were cut off, the egg developed into 
‘a coum larva, which was devoid of the apical 
j4ute and apical tuft of cilia and also of mesoderm, ‘and 
of the whole post-trochal region. If the second polar 
lobe were cut off, a trochophore larva was formed, 
provided with apical plate and apical tuft, but devoid, 
as before, of mesoderm and of post-trochal region. 
The conclusion was inevitable that the specific material 
for the apical plate and posttrochal region was con- 
tained in the first polar lobe, but that the second polar 
lobe only contained the necessary material for the post- 
trochal region. 
Driesch’s objections to this conclusion were founded 
on thé difficultv of conceiving what an. organ-forming 
substance could be like, it being very difficult to pic- 
ture a substance the molecules of which had the power 
of ‘‘crystallising"’ into organs, such as, ‘for instance, 
arms and legs. But if we fell back on our ultimate 
conception of what we meant by ‘ explanation,” we 
found that it alwavs consisted in comparing a less fami- 
NO. 2450, VOL. 98] 
liar phenomenon with one about which we thought we 
knew more. Driesch’s enteiechy was really an attempt 
to compare the forces which organise development with 
the purpose of an intelligent being who wanted to 
build a house, and, in principle, no fault could be 
found with it. ‘he great dithculty about it was that 
this comparison does not help us to understand in the 
least a large number of phenomena which could be 
tar better ~“explained’’ by the theory of organ-form- 
ing substances, even although we could not tell what 
these substances were like. 1n the development of the 
Ascidian, Cynthia partita, as described by Conklin, 
the cytoplasm was rendered slaty-blue by inclusions of 
yolk, and in its outermost zone were numerous par- 
ucles of bright yellow pigment. Before fertilisation the 
large germinal vesicle burst, and its contents formed a 
cap of clear fluidat one pole of the egg. ‘Lhe spermato- 
zoon entered at the opposite pole, and then the clear sub- 
stance and the yellow pigment were drawn down to 
meet it, and eventually tormed two concentric crescents 
round the lower pole of the egg. Subsequent develop- 
ment made it plain that the clear substance gave rise 
to the ectoderm, the slaty-blue cytoplasm to the endo- 
derm, and the yellow material to the mesoderm of the 
Ascidian tadpole. If one of the first four cells of the 
segmenting egg were killed, the other three continued 
their development, and an imperfect embryo was pro- 
duced; if this cell happened to be one of the two 
containing yellow substance, a tadpole was produced 
which had muscles only on one side of its tail. 
Clearly in this case the organ-forming substances were 
visible to the naked eye, since they were distinguish- 
able in colour, and their segregation in different regions 
of the embryo was the real cause of the differentiation 
of the germ-layers. In this process the individual cell 
was not a unit of any importance; both notochord and 
nerve-cord arose from the same group of cells, termed 
by Conklin chorda-neural cells; but the cytoplasm of 
these cells consisted of clear and blue portions, and in 
the subsequent divisions the clear portions were added 
to the ectodermic neural plate, whilst the blue portions 
became the endodermic notochord. 
Driesch explained phenomena like these by asserting 
that these substances were the conditions, not the 
causes, of the development of organs; but another 
experiment, due to Morgan, which had been repeated in 
the laboratory of the Imperial College of Science, 
appeared to dispose completely of the idea of there 
being an intelligent entelechy presiding over develop- 
ment. This experiment consisted in fastening frogs’ 
eggs to a slide, with the black pole uppermost, and 
fertilising them in this position. When the eggs had 
divided into two another slide was laid on the top of 
them and clamped in this position; the whole prepara- 
tion was then inverted and allowed to develop for five 
or six days in this position. At the conclusion of this 
period a double-headed, or double-tailed tadpole. was 
produced. In this case nothing was added to the egg, 
but the dark substance, which was specifically lighter 
than the white substance which constituted the 
rest of the egg, had TYreadjusted itself im each 
cell under the influence of gravity, in a similar manner 
to what it would have done in the whole egg if this had 
been inverted before division into two had taken place. 
Hence the condition of the formation of a frog embryo 
must be the proper spatial relationship between two 
organ-forming substances. ; 
If we adopted the view that organ-forming substances 
were the all-important agents in development, it be- 
came of the utmost importance to learn more about 
them. Observation of the developing egg of Ascaris 
showed that the relative proportion of such a substance 
in one cell as compared with its quantity in a neigh- 
bouring cell could determine the fate of the cell. This 
egg divided into two cells at its first cleavage, one of 
