76 



THE BIOLOGY OF TWINS 



bryos. They would also never establish a nutritive con- 

 nection with the placenta. 



A drawing of a wax model (Fig. 27) made from an 

 ectodermic vesicle from which numerous embryonic 

 outgrowths are being given off shows two outgrowths 

 that give promise of rudimentation. Embryonic out- 

 growth 9 has evidently been produced too high up or 

 too close to the original apical end of the vesicle and 



Fig. 31. — A group of primitive streak embryos of D. hyhridus show- 

 ing how some of the ectodermic outgrowths {F and H) fail to secure 

 attachment to the endodermic parts of the egg. This may account 

 for the rudimentary embryos seen in Figs. 29 and 30. (After Fernandez.) 



has not been able to grow out as rapidly as the rest; 

 embryo 2 is small and apparently subsidiary to i and 

 would probably have been rudimentary. Rudimentary 

 embryos that are no more than formless masses of 

 tissue attached by amniotic connecting canals with 

 the common amnion are shown in Fig. 29. Here also 

 are shown marked irregularities in the interrelations 

 of embryos, as indicated by the forking of amniotic 

 canals. 



