PRODUCTION OF TWINS AND DOUBLE-MONSTERS 343 



which is that each side acts independently of the other and 

 produces all of the structures characteristic of the species. 

 Neither side is in these circumstances a subordinate or inhibited 

 side, but both sides are of equal dominance. From such larvae 

 as these there arise, without question, the occasionally observed 

 adults with double madreporic plates and stone canals. A more 

 detailed account of these double-pored larvae is given in a sep- 

 arate paper (Newman, '20 b). 



Summary of data on twins in Patiria 



1 . For present purposes all forms of larvae are classed as twins 

 in which doubling of normally single structures occurs; for 

 twinning is believed to be, in last analysis, a process involving 

 duplication of originally single structures. 



2 Several distinct types of twinning are found in Patiria 

 larvae, of which the following is a list: 



a. Completely separated half- and quarter-sized blastulae, 

 derived through physiological isolation of the blastomeres of the 

 two-cell or of the four-cell stages. 



b. Partially separate blastulae, double monsters, derived 

 through incomplete physiological isolation of the blastomeres of 

 the two-cell stage. 



c. Double-monster gastrulae derived by gastrulation of forms 

 described in b. 



d. Gastrulae with two, three, or more archentera, derived 

 from blastulae in which the axial gradient or general polarity 

 had been either eliminated entirely or had been radically dis- 

 turbed. 



e. Advanced gastrulae or bipennariae in which the distal 

 parts of the paired archentera have undergone fusion and in which 

 the proximal parts of the archentera and the blastopores remain 

 separate. 



/. Advanced gastrulae or bipennariae in which the paired 

 archentera remain separate throughout life, involving the differ- 

 entiation of two sets of alimentary derivatives. 



