EXPERIMENTAL PRODUCTION OF TWINS 13 



long unanswered. One needed only to watch carefully 

 a batch of unfertilized eggs simultaneously with another 

 batch of fertilized eggs to note certain obvious differences: 

 (a) Both sets of eggs undergo maturation at the same 

 time, (b) Numerous eggs in the fertilized lot produce 

 well-elevated fertilization membranes; while in partheno- 

 genetic cultures this happens in only a few eggs, {c) In 

 two hours after insemination the fertilized eggs begin 

 cleavage, which goes forward at a rapid rate; while in 

 the unfertilized lot those eggs that had formed a mem- 

 brane fail to cleave and disintegrate after a few hours. 

 (d) Some of the maturated eggs in the unfertilized lot, 

 without having formed any distinctly lifted-up fertiliza- 

 tion membranes, begin cleavage after about six hours, 

 nearly four hours later than in the fertilized eggs, and 

 cleavage goes on relatively much more slowly than in 

 the latter. 



It is obvious from these observations that the 

 development of parthenogenetic eggs has been greatly 

 slowed down. The stimulus afforded by the entrance 

 of the spermatozoon has been lacking, as is evidenced 

 by the absence of cortical changes and the failure of a 

 fertilization membrane to be elevated from the vitelline 

 membrane. Moreover, all of the developmental changes 

 go on at an abnormally slow rate. 



B. Hybrid twins. — -Twins of another sort, rather 

 better developed and more viable than parthenogenetic 

 twins, were frequently found in cross-bred cultures in 

 which the eggs of Patiria had been fertilized by the sperm 

 of various other species of starfishes and sea urchins. 

 As compared with pure-bred Patiria larvae, hybrid 

 larvae always showed a very distinctly slower develop- 



