340 THE FOKMS OF TISSUES [ch. 



the egg of Trochus (a marine univalve mollusc) proceeded in 

 accordance with the laws of surface tension, but he also succeeded 

 in imitating by means of soap-bubbles, several stages, one 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 earlier stages the experiment is 

 easy ; we have merely to blow our bubbles in a little 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 t^ie "polar furrow" of which we have spoken on p. 310. 

 On blowing four little 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 little above the others, and which 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. 135, 1-3). The lamina itself is plane when the system is 

 symmetrical, but it responds by a corresponding curvature to 

 the least inequality of the bubbles on either side. In the 

 experiment, the upper polar furrow is usually a little 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 

 at|B.chments to the sides of the dish (Fig. 135, 4). A similar 

 phenomenon is usually found in Trochus, according to Robert, 

 and many other observers have likewise 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, that 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 allied species. 



