EXPERIMENTAL PRODUCTION OF TWINS 



19 



striking example of the way in which a stronger twin 

 may dominate over and ultimately rid itself of a weaker 

 twin so as to come to be practically a normal individual. 

 Occasionally also two quite equivalent paired archentera 

 grow toward one another at such an angle that they meet 



Figs. 7, 8. — Two advanced larvae of Patiria. Fig. 7, a symmetrical 

 twin with the two separate alimentary tracts considerably differentiated 

 and exhibiting striking mirror-image symmetry. Fig. 8, a twin of the 

 same age with the anterior ends of the two alimentary tracts fused to 

 form a common pharynx. (From Newman.) 



in the middle of the cleavage cavity and fuse at their 

 anterior ends. Such larvae come to have one anterior 

 and two posterior archentera and two blastopores. 

 In several cases such larvae as this (Fig. 8) went so 

 far as to differentiate two stomach enlargements, two 

 oesophagi and two intestines, cornmunicating with a 

 common pharynx. 



3. Twins with bifurcated archenteron. — A relatively 

 rare type of retarded larva undergoes dichotomy of the 

 anterior end of the archenteron, which results in a sort 

 of '^double-headed" condition. This phenomonen is 



