12 THE PHYSIOLOGY OF TWINNING 



derm, but I hardly expected to find so favorable a form 

 as Patiria, a form that produces abundant twins almost 

 without any effort on the part of the investigator. 

 Under ordinary laboratory conditions, and especially 

 under conditions of crowding, one gets a high percentage 

 of twins of many sorts. It then becomes the task of the 

 investigator to discover the factors responsible for this 

 high incidence of twins. Such an analysis has been made 

 and is here to be presented. 



THE CONDITIONS UNDER WHICH TWINS 

 ARE PRODUCED 



A. Partheno genetic twins. — -It was a matter of some 

 surprise to note in certain control cultures of Patiria 

 eggs, that had never been fertilized, considerable num- 

 bers of actively swimming larvae. Many of these larvae 

 were of dwarf size, others were almost formless, wrinkled 

 ciliated masses; but there were always some distinctly 

 twinned larvae, in the sense that they had two or more 

 archentera or primitive alimentary tracts. Such twins 

 lived a long time but were always quite subnormal with 

 respect to the development of typical larval characters. 

 The percentage of parthenogenetically developing eggs 

 in Patiria ranges from o in some cultures to as high as 

 75 in one culture. The usual percentage ranges from 

 i to lo and is more commonly about 2 or 3. Of the eggs 

 developing parthenogenetically as many as a third, or 

 possibly a half in some cases, show some phase or other 

 of tv^ inning. 



What is there about parthenogenesis that favors 

 twinning? This was, of course, the most obvious 

 question that presented itself, and it was not to remain 



