GIANT LARVAE FORMATION IN ARBACIA PUNCTULATA. 29 



In the next example also, the axes tended to approach a parallel 

 position, but the blastopores were 180 degrees apart. The two 

 gastrulae had their axes originally about 60 degrees apart (Fig. 46). 

 The next day with the development of both retarded, their axes 

 had shifted to about 180 degrees apart and their size and degree 

 of differentiation were more unequal (Fig. 47). This pair de- 

 veloped into a "single" somewhat irregular larva. 



Fig. 48 is another instance of axial rotation towards the zero 

 point. This figure is a free-hand drawing of two nearly equal 

 gastrulae whose axes were 180 degrees apart. The next day 

 the axial angle had shifted about 75 degrees. This angle was 

 maintained throughout subsequent development. The resulting 

 double larva is shown in Fig. 49. 



It will be evident even from these few .examples that the 

 relative position of the axes may be, and had in fact been shifted 

 during early development of the gastrulae; that the shifting took 

 place towards or away from blastomeric symmetry of the axes, 

 and that the range of movement was surprisingly large, from 

 about 20 to 1 80 degrees on the one hand and from 180 to o 

 degrees on the other. 



It is conceivable that with such shifting of the axes, an original 

 asymmetrical pair may subsequently become symmetrical, and 

 we should then expect from Boveri's hypothesis that development 

 would then be single and Riesenlarva would result. But such 

 Riesenlarva may or may not come out of such symmetrical 

 pairs. That Riesenlarva may be formed in this way cannot be 

 denied. But that there is any necessary relation between sym- 

 metrical position of the axes and Riesenlarva formation, is 

 extremely doubtful in view of the examples mentioned above. 



On the other hand, if symmetry is not established either by 

 rotation of the axes or by the original position of the pair, we 

 should not anticipate the formation of single larvae. But as I 

 have shown, such results do occur in at least the four cases 

 described in Figs. 30 to 32, 26 to 29, 15 to 16, 12 to 14. 



There is of course the possibility that while the axes may be 

 parallel and symmetrical yet the planes through these axes 

 may not have been parallel, and as de Hahn points out correctly, 

 in the absence of symmetry of the planes, the two members are 

 not really blastomerically symmetrical. 



