PRODUCTION OF TWINS AND DOUBLE-MONSTERS 341 



b. An entirely new type of twin larva was noted in two inde- 

 pendent cultures of normally fertilized eggs. On the fourth or 

 fifth day after fertilization it is noted that many larvae exhibit 

 a forking of the anterior end of the archenteron. This forking 

 is sometimes very incomplete, the anterior end of the archenteron 

 merely flaring out and becoming hammer-headed. Many stages 

 of forking (figs. 43, 44, 45) far more complete than this occur, 

 however, and in the more extreme cases it becomes perfectly 

 obvious that the archenteron has undergone fission and that 

 two distinct anterior ends result. In several cases each of these 

 'heads' formed paired hydro-enterocoel pouches, both right and 

 left pouches being distinct on both components (fig. 45). In 



Figs. 43, 44, and 45 Examples of the so-called 'dicephalous' larvae 



other cases the outside pouches were distinct, but the inside ones 

 were incompletely separated (fig. 44) or entirely fused into a 

 single median pouch (fig. 43). Out of a large number of these 

 'dicephalous' larvae only a very few lived beyond the sixth day, 

 and in no case was there any considerable progressive differen- 

 tiation beyond that just described. 



c. A third type of anomalous larva about which there is still 

 some question, but which I believe belongs to the same series 

 and is due to the same type of cause as the twinning types just 

 mentioned, is the advanced bipinnaria larva with paired madre- 

 poric pores and pore canals. In one apparently quite healthy 

 culture of larvae from normally fertilized eggs of Patiria were 

 found twenty-seven anomalous specimens in which there were 



