124 H - H - NEWMAX. 



larvae in general to continue to maintain their original bilaterality. 

 That is a different problem altogether. 



3. A third theory involves the idea that these duplicated struc- 

 tures are the products of twinning and this is what I believe they 

 are. In laboratory cultures of Asterina (and in at least two 

 other species of asteroids with which I have worked) there are 

 numerous instances of twins and double monsters. The exact 

 cause of twinning and doubling is not fully known and will not be 

 discussed here, as I have in preparation a more extensive paper 

 on this subject. It may be said, however, that a long series oi 

 types has been found in which the original right and left pri- 

 mordia of the future embryo become more or less completely 

 physiologically isolated, and in proportion to the duration or com- 

 pleteness of the isolation, each half develops more or less inde- 

 pendently of the other. The result is a series of more or less 

 completely doubled larvae, as follows : twin blastulae which soon 

 separate and develop into half-sized larvae ; gastrulse with paired 

 archentera ; gastrulae with paired blastopores, but with archen- 

 tera fused anteriorly ;. early bipennariae with the anterior end of 

 the archenteron branched and with the beginning of a double set 

 of hyclroenterocoel pouches ; and finally advanced larvae with 

 double madreporic pores and water canals. I have placed the 

 type of anomalous larvae under discussion at the end of what 

 appears to me to be a logical series, representing the results of 

 varying degrees of physiological isolation of bilateral halves of 

 embryos. The type of result attained seems to depend on the 

 time of incidence of the cause or causes of the physiological 

 isolation in question and upon the degree of severity or duration 

 of the causal agent, whatever it may be. 



There is therefore no more justification for the use of "double- 

 pored" larvae of echinoderms as evidence of an ancestral condi- 

 tion than there is for giving a similar significance to instances of 

 dicephaly or spina bifida in vertebrates. For they are all, in my 

 opinion, phases of "twinning" in the broad sense and will doubt- 

 less be explicable on some general physiological basis that we 

 shall hope to discuss in the future. Let me close with a word of 

 caution. Beware of giving a phylogenetic interpretation to an 



