VIl] 



OF POLAR FURROWS 



341 



Returning to the experimental case, Robert found that by with- 

 drawing a Uttle air from, and so diminishing the bulk of the two 

 terminal bubbles (i.e. those at the ends of the intermediate lamina), 

 the upper polar furrow was caused to elongate, till it became equal 

 in length to the lower; and by continuing the process it became 

 the longer in its turn. These two conditions have again been 



Fig. 135. Aggregations of four soap-bubbles, to shew various arrangements of 

 the intermediate partition and polar furrows. (After Robert.) 



described by investigators as characteristic of this embryo or that ; 

 for instance in Unio, Lillie has described the two furrows as 

 gradually altering their respective lengths* ; and Wilson (as Lillie 

 remarks) had already pointed out that "the reduction of the 

 apical cross-furrow, as compared with that at the vegetative pole 



* F. R. Lillie, Embryology of the Unionidae, Journ. of Morphology, x, p. 12, 

 1895. 



