GIANT LARVAE FORMATION IN ARBACIA PUNCTULATA. 3! 



or rate of development, or possibly in vigor or metabolism, is 

 almost certain to be followed by the train of events just enu- 

 merated. 



It might still be urged that Riesenlarva are formed only when 

 the eggs are blastomerically symmetrical at the moment of their 

 agglutination or fusion; that if the fused pair become sym- 

 metrical at any later time, as was shown to be possible, such 

 symmetry was of no avail. But since there is no definite means 

 of determining the polarity of the sea urchin eggs at the time 

 they are definitely fused together, no positive data can be given 

 for or against this possibility, with this material. 



Several instances were shown in which asymmetry from the 

 earliest observable moment was followed by single larva forma- 

 tion, and symmetrical pairs did or did not result in single larvae. 



And finally a word may be said about the polarity of the 

 grafted members. That there is a shifting of the axes, involving 

 the body wall, the gut and the skeleton, has already been pointed 

 out. That mesenchymal and endodermal cells may be shifted 

 was shown by Driesch and Goldfarb, but I cannot translate these 

 changes as effecting the polarity of either member. 



Following the absorption of the connecting ectodermal wall 

 between the body cavities of the two members, the pair tend 

 increasingly to reach a state of form equilibrium. This involves 

 the migration of the mesenchyme and endodermal cells, with the 

 consequent change in position and shape of the several structures. 

 There is a mechanical shifting of some cells but not a change of 

 polarity of the cells or of the embryo. 



SUMMARY AND CONCLUSIONS. 



1. The development of each pair of fused blastulae of the 

 sea-urchin Arbacia punctulata, was studied separately, and some 

 of these pairs gave rise to single larvae (Riesenlarvae). 



2. Such larvae are not consequent upon blastomeric symmetry 

 of the two fusing members, as urged by Boveri and de Hahn. 



(a) The axis of each member is first definitely known at the 

 gastrula stage. 



(&) One or both members may differentiate their guts and 

 thereby establish the axes. 



