EXPERIMENTAL PRODUCTION OF TWINS 23 



indeed, the whole organization of the embryo. The 

 old egg symmetry largely passes away and a new sym- 

 metry of the embryo and larva takes its place. In the 

 echinoderms, of course, even this new antero-posterior 

 axis and bilateral symmetry are largely done away with 

 when the larva undergoes metamorphosis into the 

 radially symmetrical adult. This, however, does not 

 concern us in the present connection. 



SECONDARY PHASES OF GASTRULATION 



Although, as has been said, the initial steps of 

 gastrulation involve an inpushing of a relatively basal 

 region characterized by low metabolic rate, a new 

 apical point soon arises at the distal or ingrowing end 

 of the archenteron. Such a region becomes, in a sense, 

 the head end of the larva and is the most actively 

 growing and differentiating region of the latter. In a 

 few cultures of Patina there occurred larvae reminding 

 one strongly of common types of double-headed human 

 and fish monsters known technically as anadidymi, 

 individuals divided anteriorly and united posteriorly. 



ANADIDYMI AMONG STARFISH LARVAE 



These larvae occurred in cultures that had been 

 normally fertilized and in which the great majority of 

 individuals were quite typical. What had been the cause 

 of their occurrence was not clear. These larvae, how- 

 ever, were obviously retarded forms, for they were 

 considerably smaller and less active than normal larvae. 

 They seem to have developed normally until they had 

 reached a late gastrula stage, a stage when it may be 

 said that the axis of symmetry had been well established. 

 Then, through a dichotomy of the anterior end of the 



