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the magnesium salts may be eliminated, and we may take up sea 

 water containing 1,000 parts of sodium chloride, 22 parts of 

 chloride of potash, and 10 parts of chloride of lime, and the sea 

 urchins will develop in that perfectly normally. In order to produce 

 twins by causing these two cells to fall apart, it is only necessary 

 to eliminate any one of these constituent*. Leave out the lime 

 and the embryos which develop in that solution are always twins. 

 Leave out the potash and you get the same result. Leave out 

 the sodium and you get the same result. 



Twins which are produced by mammalia, to which class of 

 animals we ourselves belong, are always one of two kinds. They 

 may be dissimilar twins which are easy to distinguish, and may 

 be of different sexes. They are probably produced from two 

 separate egg cells, but there is another class of twins about which 

 the stories are always told of blue ribbon and red ribbcn being 

 used to distinguish them — Wins who resemble each other most 

 strikingly, and are always of the same sex. These twins are not 

 improbably produced by the falling apart of the two first cells into 

 which the egg cell divides. Probably there is a slight disturbance 

 in the composition of the blood. The blood in the body, so far as 

 the inorganic salts are concerned, approximates very closely to. the 

 composition of the sodium, potassium, and lime mixture which 1 

 have just described, and a slight disturbance might easily result 

 in the falling apart of these two cells, and result in the production 

 of identical twins. 



Ten or twelve years ago Professor Loeb set himself the pro- 

 blem of artificially fertilizing the egg of some animal without the 

 assistance of spermatozoa. In the search for this method it is 

 unquestionable that he was guided by a principle which he has 

 probably forgotten to-day. An hypothesis when it has led us to 

 facts, and is no longer serviceable, is not worth remembering. One 

 of the earliest things he tried was simply to increase the concen- 

 tration of sea water. We can do that very easily in a variety of 

 ways. We can evaporate it to a certain extent. Of course, if 

 you go too far the salts crystallize out, but you can evaporate a 

 little and still keep the salts in solution, or you can add to sea 

 water a salt solution or a sugar solution of higher concentration 

 than itself. When we have prepared such a sea water concen- 

 trated about 60 per cent., and placed in it sea- 

 urchin eggs, the following phenomena occur. The egg is left in 

 the concentrated water for something like two hours, and is then 

 taken out and put intc normal sea water. Within half an hour 

 or so after that, the cell divides and so forth just as the normal 

 fertilized eggs do, but it does not produce that clear membrane 

 which is characteristic of normal fertilization. When the embryo 

 is produced — a free-swimming embryo of pyramidal shape, which 

 swims by means of cilia, and possesses a skeleton — instead of 

 swimming on the surface of the water like a normal embryo it 

 swims at the bottom of the water. Obviously at that time Pro- 

 fessor Loeb had not succeeded in completely imitating natural 

 fertilization, but he had made a long stride towards doing so. 



Let us analyse a little more fully this fertilization by increased 

 concentration. It has been found by later experimentation that 

 in this phenomenon two factors are concerned. One is the con- 

 centration of the sea water. At first it was imagined that this 

 was the only factor, but we now know there is another, and that 

 is the quantity of alkali in the sea water. Most sea water is 

 very faintly alkaline, behaving as though it had a very slight 



