DEVELOPMENT OF TDEKTICAL' TWINS 133 



It has been long known that twins of the same sex 

 are often enclosed in the same embryonic membranes, 

 while twins of opposite sex are always enclosed in 

 separate membranes. The latter would be the product 

 of distinct ova, which had been separately fertilized, as 

 in the ordinary multiple births of animals (cats, dogs, 

 rabbits, &c.). The former would be the product of 

 a single ovum, which has divided into two ova, in all 

 probability after fertilization. But it is clear that the 

 ova arising from the two halves of a single ovum, at 

 a time when the individual characteristics were already 

 determined, would be very nearly identical : their resem- 

 blance would be of a very different order from that of 

 separate ova. We also find that some twins of the 

 same sex present resemblances of a very different order 

 from that of brothers or sisters who are developed from 

 separate ova. It must be admitted, therefore, that there is 

 a very high degree of probability that the ' identical ' ova 

 are those which develop into the ' identical ' individuals. 

 The interesting conclusion that sex is predetermined in 

 the fertilized ovum also follows from the same facts. 



The probable beginning of the development of * iden- 

 tical ' twins is shown in Diagram ///. A* is a fertilized 

 ovum with the individual characteristics predetermined. 

 At its first division A* does not, like A in Diagram //, 

 form the cells of Stage 2, indicated by the letters B 

 and C ; but it divides without differentiation into two 

 equivalent cells, like each other and like the ovum. 

 Hence the first division of A* does not produce Stage 2, 

 but Stage i'^, consisting of two similar ova. Each of 

 these then divides, as shown in Diagram ///, forming 

 a true Stage 2, comparable to that of Diagram //. 

 After this the other stages succeed as in the latter, 

 and finally two individuals will be formed, which must 

 resemble each other if it be true that individual charac- 

 teristics are predetermined in the fertilized ovum. And, 

 as a matter of fact, such resemblances are seen in indi- 

 viduals whose development may be considered, with a 

 very high degree of probability, to have followed the 

 lines indicated in Diagram ///. 



