74 



THE BIOLOGY OF TWINS 



Fig. 27. — Drawing of a wax model of 

 the ectodermic vesicle of an egg of D. 

 hybridus, showing the relationship of the 

 nine embryonic outgrowths. Note the 

 great irregularity and lack of definite 

 pairing, (From Fernandez.) 



embryos have arisen; from the other only one rudi- 

 mentary or degenerate embryo. Other arrangements of 



embryos are shown 

 in Figs. 28 and 29. 



This highly vari- 

 able condition in the 

 Mulita is in sharp 

 contrast with the 

 rather definite one 

 that prevails in D. 

 novemcinctus, where 

 about 97 per cent of 

 sets have four fetuses. 

 One cannot help 

 suspecting, however, 

 that, as was previ- 

 ously suggested, the 

 quadruplet condition 

 in the last-named 

 species is not always 

 arrived at in the 

 same way. Just as in 

 the Mulita one sprout 

 may remain single 

 and another sub- 

 divide once or several 

 times, so in our spe- 

 cies it may well be 

 that quadruplets 

 arise, not always in 

 pairs, but sometimes three on one side and one on the 

 other. This would furnish an explanation for the con- 



FiG. 28. — Showing the same region of 

 D. hybridus that is shown for D. novem- 

 cinctus in Figs. 21 and 22. Note the very 

 irregular interrelations of the connecting 

 canals of the embryos yl to /. C connects 

 with a rudimentary embry^o. A and B 

 might be called a pair; similarly D and E. 

 (From Fernandez.) 



