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 1 ." 



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 a 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, Arehivf. Entwickelungsmechanik, Vol. 23, p. 249, 1907. 



