PEODUCTION OF TWINS AND DOUBLE-MONSTERS 323 



This theory met with a degree of coolness and skepticism, 

 partly, no doubt, owing to the fact that it was based upon 

 Child's 'axial gradient theory,' a far-reaching and consistent 

 principle, which, though at first coldly received or ignored by 

 American biologists, is slowly but surely coming into its own. 

 It has been the writer's desire for some years past to test out 

 this physiological theory of twinning experimentally. The arma- 

 dillo material is too difficult to secure or to manage. It is there- 

 fore advisable to work with some more available material. When 

 making plans for a period of several months' work at Pacific 

 Grove, California, the writer hoped to encounter some marine 

 material that would be suitable for twinning experiments, and 

 was fortunate to find in the starfish Patiria miniata a species 

 almost ideal for this purpose. 



MATERIAL AND METHODS 



Patiria miniata is a common starfish of the California coast. 

 It belongs to the first order of the subclass Asteroidea, viz., 

 Spinulosa. This is the same species which Loeb used for ex- 

 periments on artificial parthenogenesis and to which he applied 

 the name Asterina. For most purposes Patiria is a decidedly 

 unfavorable species, because it is so difficult to obtain eggs in 

 prime or uniform condition. Attempts to cause the extrusion 

 of ripe eggs from the genital pores have been unavailing, and 

 the somewhat crude method of gently shaking the carefully 

 excised ovaries in sterile sea-water has been adopted as a means 

 of obtaining eggs for experiment. For most purposes this method 

 furnishes too large a percentage of immature eggs and too wide 

 a range of diversity in the state of ripeness of the full-grown 

 oocytes. Of the eggs that appear to be fully mature, a large 

 though varying per cent never undergo maturation; some that 

 maturate form membranes simulating fertilization membranes, 

 but do not undergo parthenogenetic cleavage; usually a small 

 percentage of eggs that maturate and do not form a membrane 

 undergo parthenogenetic cleavage; while the majority of the 

 maturated eggs neither form a membrane nor undergo cleavage 

 unless fertilized either by sperm of the same species or by that 



