VII] OF POLAR FURROWS 557 



of Surface-tension, but he also succeeded in imitating by means of 

 soap-bubbles one stage after another of the developing egg. 



M. Robert carried his experiments as far as the stage of sixteen 

 cells, or bubbles. It is not easy to carry the artificial system quite 

 so far, but in the earHer stages the experiment is easy; we have 

 merely to blow our bubbles in a httle dish, adding one to another, 

 and adjusting their sizes to produce a symmetrical system. One 

 of the simplest and prettiest parts of his investigation concerned 

 the "polar furrow " of which we have spoken on p. 489. On blowing 

 four httle contiguous bubbles he found (as we may all find with the 

 greatest ease) that they form a symmetrical system, two in contact 

 with one another by a laminar film, and two which are elevated 

 a httle above the others and are separated by the length of 

 the aforesaid lamina. The bubbles are thus in contact three by 

 three, their partition-walls making with one another equal angles 

 of 120°. The upper and lower edges of the intermediate lamina 

 (the lower one visible through the transparent system) constitute 

 the two polar furrows of the embryologist (Fig. 215, 1-3). The 

 lamina itself is plane when the system is symmetrical, but it responds 

 by a corresponding curvature to the least inequahty of the bubbles 

 on either side. In the experiment, the upper polar furrow is usually 

 a httle shorter than the lower, but parallel to it; that is to say, 

 the lamina is of trapezoidal form : this lack of perfect symmetry 

 being due (in the experimental case) to the lower portion of the 

 bubbles being somewhat drawn asunder by the tension of their 

 attachments to the sides of the dish (Fig. 215, 4). A similar 

 phenomenon is usually found in Trochus, according to -Robert, 

 and many otlier observers have hkewise found the upper furrow 

 to be shorter than the one below. In the various species of the 

 genus Crepidula, Conklin asserts that the two furrows are equal 

 in C. convexa, ^'^hat the upper one is the shorter in C. fornicata, 

 and that the upper one all but disappears in C. plana; but we 

 may well be permitted to doubt, without the evidence of very 

 special investigations, whether these slight physical differences are 

 actually characteristic of, and constant in, particular species. 

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

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

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



