i8o THE PHYSIOLOGY OF TWINNING 



as on the left. This appearance of bilateral symmetry 

 in the occasional larva has been interpreted by McBride 

 and others as a reversion to an ancestral state, and is 

 beheved strongly to support the theory that the echino- 

 derms, at present a radially symmetrical group, have 

 been derived from a bilaterally symmetrical ancestor 

 through the gradual suppression of the coelomic struc- 

 tures of the right side. 



Recently the writer (1921, a), while engaged upon 

 the experimental production of twins in the starfish 

 Patiria miniata discovered a culture of advanced bipen- 

 nariae in which the majority of individuals had both 

 right and left madreporic pores and pore-canals. A 

 typical specimen of this culture is shown in Figure 64. 

 These specimens were nearly always strictly bilaterally 

 symmetrical, but a few showed a slightly less perfect 

 condition of the madreporic canal or pore on the right 

 side than on the left. It should be emphasized that these 

 bilaterally symmetrical larvae were found in a crowded 

 culture in which development had taken place under 

 somewhat subnormal conditions. It previously had 

 been found that when large numbers of eggs were allowed 

 to develop in a single vessel there always occurred a 

 considerable number of various types of twins. When, 

 therefore, along with a considerable number of twin 

 larvae, there appeared a great many of these bilaterally 

 symmetrical larvae, it was natural to look upon this 

 condition as a kind of twinning. The idea then was that 

 any doubling of normally single structures might be 

 interpreted as twinning, and, therefore, animals with 

 paired madreporic pores and pore-canals, instead of the 

 single pores and canals, were essentially twins. I now 



