VII] OF LAMARLE'S LAW 487 



must De able to view the object in a plane at right angles to the 

 boundary walls. For instance, in any ordinary plane section of a 

 vegetable parenchyma, we recognise the appearance of a "froth," 

 precisely resembling that which we can construct by imprisoning 

 a mass of soap-bubbles in a narrow vessel with flat sides of glass; 

 in both cases we see the cell-walls everywhere meeting, by threes, 

 at angles of 120°, irrespective of the size of the individual cells: 

 whose relative size, on the other hand, determines the curvature of 

 the partition-walls. On the surface of a honey-comb we have 

 precisely the same conjunction, between cell and cell, of three 

 boundary walls, meeting at 120°. In embryology, when we examine 

 a segmenting egg, of four (or more) segments, we find in like manner, 

 in the majority of cases if not in all, that the same principle is still 

 exemplified. The four segments do not meet in a common centre, 

 but each cell is in contact with two others ; and the three, and only 

 three, common boundary walls meet at the normal angle of 120°. 

 A so-called polar furrow"^, the visible edge of a vertical partition- 

 wall, joins (or separates) the two triple contacts, precisely as in 

 Fig. 172, B, and so gives rise to a diamond-shaped figure, which 

 was recognised more than a hundred years ago (in a newt or 

 salamander) by Rusconi, and called by him a tetracitula. 



That four cells, contiguous in a plane, tend to meet in a lozenge 

 with three-way junctions and a "polar furrow" between the cells, is 

 a geometrical theorem of wide bearing. The first four cells in 

 a wasp's nest shew it neither better nor worse than do those of 

 a segmenting ovum, or the ambulacral plates of a sea-urchin 

 or the oosphere of Oedogonium giving birth to its four zoo- 

 spores!. Going farther afield for an illustration, we find it in the 

 molecules of a viscous liquid under shear: where a group of four 



* It was so termed by Conklin in 1897, in his paper on Crepidula (Journ. Morph. 

 XIII, 1897). It is the Querfurche of Rabl (Morph. Jahrh. v, 1879); the Polarfurche 

 of (). Hertwig (Jen. Zeitschr. xiv, 1880); the Brechuugslinie of Rauber (Neue 

 Grundlage zur Kenntniss der Zelle, Morph. Jahrb. viir, 1882); and the cross-line of 

 T. H. Morgan (1897). It is carefully discussed by Robert, op. rit. p. 307 seq. 



t Speaking of the complicated polygonal patterns in the test of the protozoon 

 genus Peridinium, Barrows says: "In the experience of the writer no case has 

 been found in which four sutures actually meet at one point. Cases which at first 

 sight appeared as such, upon loser analysis in a favourable position have been 

 resolved into two junction-points of three sutures each, etc." On skeletal variation 

 in the genus Peridinium, Univ. Calif. Puhl. 1918, p. 463. 



