24 THE PHYSIOLOGY OF TWINNING 



archenteron, they became distinctly ''two-headed." In 

 some of the individuals (Figs. 15, 16) the median parts 

 were quite incompletely separated and inner struc- 

 tures remained united in the middle. In others, as in 

 Figure 17, the two ''heads" became separate and only 

 the posterior part of the archenteron remained in com- 



FiGS. 15-17. — Patiria larvae in which the anterior end of the archen- 

 teron has undergone dichotomy. Fig. 15, a larva with only a slight 

 degree of dichotomy. Fig. 16, a larva with a moderate degree of dichot- 

 omy. Fig. 17, a larva with complete dichotomy of the anterior end of 

 the archenteron. (From Newman.) 



mon. For some unknown reason, larvae of this sort 

 fail to advance much farther than the stages shown, and 

 I was unable to discover the further consequences of 

 such a process of twinning. 



SECONDARY FUSIONS AND THEIR CONSEQUENCES 



As has already been said, fusions frequently occur 

 between archentera that arise closely adjacent to each 

 other. I have rarely seen a case of fusion between two 

 archentera that had arisen from distinctly separate 

 basal areas. Only when the two archentera are the result 

 of the fission of a single basal area do they exhibit a 

 strong tendency to fuse. If the two are of equal size, i.e., 

 are identical twins, they often grow together and fuse 



