PRODUCTION OF TWINS AND DOUBLE-MONSTERS 329 



It is not often that one finds four complete blastulae surrounded 

 by a single vitelline membrane, but it was possible to make 

 camera drawings of two such cases. Specimens showing two 

 blastulae within a single membrane (figs. 8 and 10) are, however, 

 relatively common, and in one instance there was observed the 

 rupture of the membrane and the total separation of the twin 

 blastulae. One fact that numerous drawings show, although at 

 first no particular significance was attached to it, is that the two 

 blastulae are seldom, if ever, alike. One of them seems to be less 

 advanced, or at least less normal, than the other. In some cases, 

 for example, one blastula is composed of smaller and more 

 numerous cells and has a larger cavity than the other. In other 

 cases one blastula is quite normal while the other is a solid 

 blastula, in which the lumen is filled with cells and the peripheral 

 cells are longer than normal. These observations tend to support 

 the conclusion, mentioned in a previous paragraph, that the later 

 bilateral asymmetry characteristic of echinoderm larvae is pre- 

 determined physiologically in the two-cell stage or even prior to 

 cleavage. If this contention is well founded, it might be possible 

 in the unsegmented egg to demonstrate for echinoderm eggs, as 

 Conklin has demonstrated for the eggs of dextrally and sinis- 

 trally spiral gastropods, a structural asymmetry of the cytoplasm. 

 Even if no visible structural asymmetry exists, there is good 

 evidence in these experiments that a physiological asymmetry 

 is present which, when exaggerated by inhibiting factors, as is 

 the case in the unequal twins just described, makes itself clearly 

 evident. 



The fate of these dwarf blastulae has not been followed out 

 with entire satisfaction, but it seems to be safe to say that only 

 the more normal of the twin blastulae ever gastrulate. A large 

 number of dwarf gastrulae have been noted, some occurring in 

 almost every lot of larvae examined, but there seem to be no 

 indications that these dwarfs progress beyond an early complete 

 gastrula stage. If they do they have escaped attention. 



The cases described in this section hitherto have dealt with 

 more or less completely isolated twin blastulae, but there are 

 frequent instances of incomplete isolation of the twins and the 



