ba 
NATURE 
[OcTOBER 12, 1916 
which produced the ectoderm and the other the in- 
ternal organs. Boveri showed that if these eggs were 
fixed to a slide which was inserted in a centrifugal 
machine and a rapid rate of rotation maintained whilst 
the egg developed, some of them the axes of which 
happened to lie exactly in the radius of rotation divided 
into two equal cells, both of which formed internal 
organs, and neither of which behaved like the cell in 
the normal embryo, which produced ectoderm. The 
slightest obliquity ot the egg axis to this radius caused 
the egg to undergo normal development. ‘This experi- 
ment, which had been repeated by us in the Imperial 
College of Science, showed that some substance was 
present in greater quantity on the outer part of the 
egg, so that the upper of the first two cells received more 
of it than the other, and was thus determined to form 
ectoderm, but that when under stress of the centri- 
fugal force the division plane separating the first two 
hlastomeres took up an exactly radial position, so that 
this substance was equally distributed to both cells, 
neither developed into ectoderm. 
The question where these substances were formed was 
of great importance. A priori considerations suggested 
that they must emanate from the chromatin. of , the 
nucleus, since the father was as potent in heredity as 
the mother, and his contribution to the zygote consisted 
merely of a mass of chromatin. This conclusion was 
confirmed both by observation and experiment. In 
the unripe egg of Cynthia, Schaxel had shown that 
‘streams of chromatin poured from the nucleus into the 
cytoplasm, and if the unripe egg of Ascaris was sub- 
jected to the most violent centrifugal force, so that it 
lost large portions of its substance, and was afterwards 
fertilised, it gave rise to a normal embryo of diminished 
size, showing that its cytoplasm was not yet organised 
as was that of the ripe egg, the different development 
of which under the stress of centrifugal force we have 
just described. The pressure experiments of Driesch 
and Hertwig, in which, by allowing eggs to develop 
in cramped positions, they disarranged the normal order 
of the nuclei, showed that the nuclei of the segmenting 
egg were alike, each possessing all the potentialities 
of the species, for these distorted eggs when relieved 
from pressure developed into normal embryos, although 
the nuclei had assumed abnormal positions, and it was 
the relative position of the substances produced by these 
nuclei, not of the nuclei themselves, which determined 
differentiation. 
periment of allowing Echinus eggs to develop in sea- 
water to which salts of lithium had been added, it 
was possible to inhibit the formation of one of these 
substances and produce an embryo consisting entirely, 
or almost entirely, of endoderm. The formation of 
these substances appeared to last for only a short 
period; after that, the nuclei appeared to be without 
formative influence on the cytoplasm, but in animals like 
Polyzoa and Ascidians which bud, this budding could be 
best explained as due to a renewed production of organ- 
forming substances by the nuclei. These substances 
were often not distributed to the formative tissues of 
the bud in the same manner as in the embryo, and 
hence the development of the bud often followed a 
different course from that initiated by the embryo. The 
**nost-generation ” of the missing half, observed by 
Roux in his half-tadpoles, and by Chun and Morten- 
sen in their half-Ctenophore larve, could be 
explained in  a_ similar way by postulating 
a renewed activity of the nuclei at the cut 
surface. Considerations of this kind were fatal to 
the conception of Weismann of the definite segregation 
of germ-cells from body-cells at the beginning of de- 
velopment, dependent on a_ differential division of 
nuclei, or, as he termed it, the formation of definite 
germ tracks. Indeed, Gatenby had lately shown that 
in the frog the supposed germ-cells which were segre- 
NO. 2450, VOL. 98] 
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Sometimes, as in Herbst’s famous ex-- 
gated at an early period of development would scarcely 
supply the needs of the first spawning season, and that 
the eggs needed for subsequent seasons were formed 
by the metamorphosis of ordinary peritoneal cells. 
Next to the discovery of organ-forming substances 
perhaps the greatest discovery in experimental embry- 
ology was the influence which the primary organs 
exerted on each other’s further development/ 
The first discovery of this influence was due 
to Herbst, who showed that if the ocular peduncle 
of a shrimp were amputated, the animal was able to 
regenerate a new one, as was the case also if the 
other limbs were cut off; but that if the optic gang- 
lion was also removed then an antenna-like organ was 
regenerated in place of an eye. From this experiment 
the conclusion was forced on us that in the normal 
development of the shrimp the ectoderm was caused 
to mould itself into the retinulz and crystalline cones 
of the eve by some influence emanating from the optic 
ganglion. This influence must be some chemical sub- 
stance emitted into the blood and comparable to the 
hormones, which we know to be emitted by organs 
like the thyroid gland, which so powerfully influence 
growth in man. Another’instance of the same thing 
was afforded by the experiments of Lewis; this observer 
cut off the optic vesicle from the brain of a young 
tadpole, and pushed the amputated organ backwards 
under the skin to .a new position; the wound healed 
up; no lens developed in the normal position, but a 
lens was developed from the skin situated over the 
optic vesicle. This experiment proved that no part of 
the skin was predestined to form the lens of the eye, 
but that any part could form the lens if acted on by 
the emanations from the optic vesicle beneath. A third 
instance was discovered from experiments in the Impe- 
rial College of Science in the rearing of the larvae of 
the sea-urchin, Echinus miliaris. In normal develop- 
ment the rudiment of the water-vascular system or 
‘“‘hydroccele’? was formed from the coelomic vesicle on 
the left side of the larva. Above it the ectoderm be- 
came invaginated so as to form the amniotic pit, from 
the floor of which were developed pointed spines and 
tube-feet, whilst beneath the hydroccele a series of 
pockets grew out from the left posterior ccelomic vesicle 
which developed into Aristotle’s lantern. On the right 
side of the larva two calcareous plates were developed 
bearing square-topped spines and pedicellarie. Under 
the influence of certain stimuli the larva could be made 
to develop a second hydroccele on its right side, and 
when this took place, from the octoderm of the right 
side and from the right posterior ecelomic vesicle respec- 
tively a right amniotic pit with spines and tube-feet 
and a right Aristotle’s lantern were developed. In 
other circumstances the formation of a hydroccele could 
be inhibited altogether, and then.calcareous plates bear- 
ing spines were formed on both sides of the larva. 
If the second or right hydroccele was small, it failed 
to inhibit the formation of plates bearing spines and 
pedicellarize proper to the right side, so that both hydro- 
coele and pedicellariz could be present together on the 
same side. The only possible explanation of these facts 
was the view that any part of the ectoderm could form 
an amniotic pit, and either left or right coelomic 
vesicles could form an Aristotle’s lantern, if acted on 
by influences emanating from the hydroccele, and 
that both sides ofthe larva were really alike in their 
constitution, and that in the total absence of a hydro- 
coele each produced calcareous plates with spines. 
The discovery of the profound influence exercised by 
the growing tissues of the embryo on one another 
lent some support to Dr. J. T. Cunningham’s theory 
of the inheritance of acquired qualities based on the 
facts known as to the influence of hormones on the 
growth of the human body. Jf it should turn out, as 
seems, from the results of these experiments, to be 
