70 



THE BIOLOGY OF TWINS 



the struggle for placental surface. What evidently 

 happens is that four or more secondary growing points 

 start to develop simultaneously, instead of the two 

 that are characteristic of D. novemcinctus, and that 

 normally each of these growing points divides into the 

 primordia of two embryos; but sometimes more than 

 two embryos are the result of this fission and sometimes 



Fig. 25. — Photographic view of a set of embryos of D. hyhridiis 

 (after Fernandez). Note the common amnion in the middle and the 

 amniotic connecting canals running to the nine embryos. 



no fission occurs. Such irregularities as these are similar 

 to the formation in D. novemcinctus of three embryos 

 instead of the typical two from one-half of the ectoder- 

 mic vesicle, resulting in five embryos. That the above 

 interpretation of the origin of the number of fetuses in 

 D. hyhridus is probably correct may be inferred from an 

 examination of Fernandez' photographs; Fig. 25 is 

 taken from one of these. Note that the amniotic 



