PRODUCTION OF TWINS AND DOUBLE-MONSTERS 337 



to the primary archenteron. A common type of larva is shown 

 in figure 31, in which the secondary archenteron is near the pos- 

 terior end, but is distinctly less advanced than the primary. The 

 extreme case of successful growth of an asymmetrical secondary 

 archenteron is shown in figure 32, where this structure is nearly 

 on a par with the primary, though coming in at right angles to 

 the main axis of the larva. 



The results of secondary fusion of archentera 



A great many cases have been noted in hybrid cultures, though 

 occasionally found both in parthenogenetic and in normally fer- 

 tilized cultures, of conjoined twins produced by the fusion of the 

 secondary archenteron with the primary. A case like that 

 shown in figure 34 is probably due to the fusion of archentera 

 in a larva that started out much like that in figure 32. It is 

 also probable that the larva shown in figure 37 is a more advanced 

 condition of the same larva that is shown in figure 34, since it 

 was found two days later in the same culture. Many other cases 

 have been noted in which there is a tendency for regulation to 

 take place in such twins, involving an absorption of the secondary 

 archenteron by the primary, and a return to a single, almost 

 normal individual. An account of this process, though inter- 

 esting from other standpoints, is scarcely germane to the present 

 discussion, and is therefore postponed for a subsequent paper. 



One type of fused twin must, however, be noted here, since 

 there is no regulation back to a single condition. In a consider- 

 able number of cases, which probably started out like the parthen- 

 ogenetic twins shown in figures 25 to 28, fusion occurs at the 

 anterior ends of the equal-sized archentera, and a new type of 

 twin results (figs 33 and 35) in which a common anterior end is 

 produced, but the posterior ends remain separate and open by 

 separate blastopores. Such larvae remain strictly symmetrical 

 and since neither component is secondary, both persist through- 

 out the life of the larva. A still different fate of secondary 

 archentera remains to be noted, in which the blastopore closes 

 and the whole structure first pinches off from the surface, as in 

 figure 36, and subsequently lies as a closed vesicle in the larval 

 body cavity. Sometimes two or more such vesicles are found. 



