Origin of twins 255 
The mechanism of the origin of twins, as the result of altering 
the composition of the sea-water, is revealed by observation of the 
first segmentation of the egg in these solutions. This cell-division is 
modified in a way which leads to a separation of the first two cells. 
If the egg is afterwards transferred back into normal sea-water, each 
of these two cells develops into an independent embryo. Since 
normal sea-water contains all three metals, sodium, calcium, and 
potassium, and since it has besides an alkaline reaction, we perceive 
the reason why twins are not normally produced from one egg. 
These experiments suggest the possibility of a chemical cause for the 
origin of twins from one egg or of double monstrosities in mammals. 
If, for some reason, the liquids which surround the human egg a 
short time before and after the first cell-division are slightly acid, 
and at the same time lacking in one of the three important metals, 
the conditions for the separation of the first two cells and the forma- 
tion of identical twins are provided. 
In conclusion it may be pointed out that the reverse result, 
namely, the fusion of normally double organs, can also be brought 
about experimentally through a change in the chemical constitution 
of the sea-water. Stockard succeeded in causing the eyes of fish 
embryos (Fundulus heteroclitus) to fuse into a single cyclopean eye 
through the addition of magnesium chloride to the sea-water. When 
he added about 6 grams of magnesium chloride to 100 cubic centi- 
metres of sea-water and placed the fertilised eggs in the mixture, 
about 50°/, of the eggs gave rise to one-eyed embryos, “When 
the embryos were studied the one-eyed condition was found to result 
from the union or fusion of the ‘anlagen’ of the two eyes. Cases 
were observed which showed various degrees in this fusion; it 
appeared as though the optic vessels were formed too far forward 
and ventral, so that their antero-ventro-median surfaces fused. This 
produces one large optic cup, which in all cases gives more or less 
evidence of its double nature.” 
We have confined ourselves to a discussion of rather simple 
effects of the change in the constitution of the sea-water upon de- 
velopment. It is @ priori obvious, however, that an unlimited 
number of pathological variations might be produced by a variation 
in the concentration and constitution of the sea-water, and experience 
confirms this statement. As an example we may mention the abnor- 
malities observed by Herbst in the development of sea-urchins through 
the addition of lithium to sea-water. It is, however, as yet impossible 
to connect in a rational way the effects produced in this and similar 
cases with the cause which produced them ; and it is also impossible 
to define in a simple way the character of the change produced. 
1 Stockard, Archiv f. Entwickelungsmechanik, Vol. 23, p. 249, 1907. 
