344 H. H. NEWMAN 



g. Middle and advanced gastrulae and early bipennariae in 

 which the originally single archenteron undergoes twinning at 

 its free or distal end, resulting in 'two-headed' forms in which 

 the two forks of the archenteron both give off hydro-enterocoel 

 pouches. 



h. Advanced bipennariae in which the madreporic pore and 

 pore canal, normally confined to the left side, is found in a fully 

 paired condition, the structures on the right being often quite 

 as well developed as on the left. 



These twin embryos and larvae are believed to represent a 

 logical series and to be the result of the same or similar causes, 

 though they are not all found under the same conditions. The 

 diversity of types is believed to be due to differences in the time 

 of onset of the causal factors and to the varying degrees of 

 severity of the inhibiting agents. 



3. Twins are found under three different conditions: a) as the 

 result of spontaneous parthenogenesis; b) as the result of hybrid- 

 ization; c) as the result of overcrowding of normally fertilized 

 eggs. 



The largest variety of twins is found in the third class, i.e., 

 from eggs normally fertilized, because there are always partheno- 

 genetic eggs in such cultures, and because, in addition to all of 

 the forms found in hybrid strains, some of the more advanced 

 strains occur only in normally fertilized cultures. Although the 

 greatest variety of twins is found in normally fertilized cultures, 

 the largest percentage of twins occurs in hybrid strains, in some 

 cases about 50 per cent of all living larvae being twins of some 

 sort. Parthenogenetic twins as a rule die early, and in no case 

 do they reach a true bipinnaria condition. Hybrid twins are 

 distinctly more viable and develop further. Twins in normally 

 fertilized cultures show the greatest viability and reach the most 

 advanced conditions. 



4. A very large percentage of the twins with two or more 

 archentera undergo regulation and become more or less normal- 

 appearing single larvae. This phenomenon has been studied in 

 detail and is to be brought out subsequently in a separate contri- 

 bution. 



