334 H. H. NEWMAN 



Summary of data on twinning in parthenogenetic cultures 



All of the larvae derived from parthenogenetic Patiria eggs 

 are to be viewed as decidedly subnormal. They show many 

 evidences of profound inhibition. A whole graded series of in- 

 hibition products are found, including abortive first-cleavage 

 stages; cases in which one blastomere of the two-cell stage seg- 

 ments and the other does not ; cases in which the two blastomeres 

 become physiologically isolated and produce conjoined or en- 

 tirely separate twin blastulae, and later conjoined or separate 

 twin gastrulae; cases of blastulae which have no polarity and 

 undergo multiple gastrulation, and cases of bipolar blastulae that 

 produce several distinct types of double-monster gastrulae with 

 two or more archentera. Never do larvae from parthenogenetic 

 eggs reach a true bipennaria stage. The larvae are always de- 

 ficient in the apical or anterior regions. There is usually a de- 

 ficiency in the size of the preoral lobe of the larva, and there is 

 seldom, if ever, any development of hydro-enterocoel pouches 

 at the anterior end of the archenteron. In a very few cases 

 larvae have been reared somewhat further than those figured. 



B. TWINS IN HYBRID PATIRIA LARVAE 



When the eggs of Patiria are fertilized in normal sea-water by 

 means of the sperm of another starfish, Pisaster ochraceus, a 

 very wide range of success in development is observed. In addi- 

 tion to all of the types described for parthenogenetic eggs, there 

 occur various other more advanced types of twin larvae and a 

 whole series of more or less normal single larvae. Not infre- 

 quently there occur larvae in these hybrid strains that appear 

 to be exactly like the advanced bipinnaria of the maternal species. 

 It is very probable, though not certain, that most of the severely 

 inhibited larvae come from eggs that have developed partheno- 

 genically. This statement is supported by the fact that even 

 in normally fertilized cultures of Patiria eggs there are found 

 eggs that behave exactly like those in parthenogenetic cultures. 

 The fact that a large proportion of the larvae succeed markedly 

 better in hybrid cultures than in parthenogenetic cultures makes 



