310 THE FORMS OF TISSUES [ch. 



Wherever we have a true cellular complex, an arrangement of 

 cells in actual physical contact by means of a boundary film, we 

 find this general principle in force; we must only bear in mind 

 that, for its perfect recognition, we must be able to view the 

 object in a plane at right angles to the boundary walls. For 

 instance, in any ordinary 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°, irre- 

 spective 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 great 

 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. 116, B. 



In the four-celled stage of the frog's egg, Rauber (an exception- 

 ally careful observer) shews us three alternative modes in which 

 the four cells may be found to be conjoined (Fig. 117). In (A) we 

 have the commonest arrangement, which is that which we have 

 just studied and found to be the simplest theoretical one; that 

 namely where a straight "polar furrow" intervenes, and where, 

 at its extremities, the partition-walls are conjoined three by three. 

 In (B), we have again a polar furrow, which is now seen to be a 

 portion of the first "segmentation-furrow" (of. Fig. 155 etc.) by 

 which the egg was originally divided into two ; the four-celled 

 stage being reached by the appearance of the transverse furrows 



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

 xm, 1897). It is the Querfurche of Rabl {Morph. Jahrb. v, 1879); the Polarfurche 

 of 0. Hertwig {Jen. Zeitschr. xrv, 1880); the Brechungslinie of Rauber (Neue 

 Grundlage zur K. der ZeUe, M. Jb. vin, 1882). It is carefully discussed by Robert, 

 Dev. des Troques, Arch, de Zool. Exp. et Gen. (3), x, 1892, p. 307 seq. 



