558 



THE FORMS OF TISSUES 



[CH. 



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 

 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 LilHe 



Fig. 215. Aggregations of four soap-bubbles, to shew various arrangements of 

 the intermediate partition and polar furrows. After Robert. 



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

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

 molluscs and annelids, 'stands in obvious relation to the different 

 size of the cells produced at the two polesf." 



When the two lateral bubbles are gradually reduced in size, or 

 the two terminal ones enlarged, the upper furrow becomes shorter 



* F. R. Lillie, Embryology of the Unionidae, Journ. Morph. x, p. 12, 1895. 

 t E. B. Wilson, The cell-lineage of Nereis, Journ. Morph. vi, p. 452, 1892. 



