DOUBLE MONSTERS IN FISHES 67 



of embryo-formation by concrescence and a secondary 

 fusion of originally separate embryonic axes, a view 

 which we have already shown to be discredited. 



How then can we explain the apparent partial sup- 

 pression of one component? Two simple explanations 

 come to mind. The first is suggested by a study of the 

 interinfluences of human one-egg twins (see chapter x). 

 The commonest mode of interinfluence which may be 

 detrimental to one or both twins is shown to have to do 

 with anastomoses of the placental blood vessels. Now 

 Coste, as long ago as 1855, showed that frequent anasto- 

 moses occur among the vitelline blood vessels of fish 

 twins and double monsters. Sometimes pronounced 

 inequalities were noted in the relative sizes of the 

 vitelline veins of the twins. The hearts of twins fre- 

 quently beat alternately so that there might occur back 

 pressures through the anastomoses. These observations 

 on fish twins seem to imply that the opportunities for 

 one twin to injure the other through the circulation are 

 as good as they are in the case of human twins; and how 

 much injury may be done in the case of the latter is 

 hereinafter abundantly shown. 



The second explanation really implies the adoption 

 of the fission theory of the origin of double monsters. 

 This theory maintains that such conjoined twins always 

 arise through the separation, more or less extensive, of 

 the two bilateral halves of a single embryonic shield. 

 It is held that the separation of the two halves of the 

 axis is due to a lowered rate of metaboHsm at the time 

 when the axis is being established, i.e., during the time 

 just preceding gastrulation. The primordium of one 

 half of the axis may, after physiological isolation from 



