312 THE FORMS OF TISSUES [ch. 



error or imperfect observation all those cases where the junction- 

 lines of four cells are represented (after the manner of Fig. 116, A) 

 as a simple cross* . 



But while a true four-rayed intersection, or simple cross, is 

 theoretically impossible (save as a transitory and highly unstable 

 condition), there is another condition which may closely simulate 

 it, and which is common enough. There are plenty of repre- 

 sentations of segmenting eggs, in which, instead of the triple 

 junction and polar furrow, the four cells (and in Hke manner their 

 more numerous successors) are represented as rounded off, and 

 separated from one another by an empty space, or by a httle drop 

 of an extraneous fluid, evidently not directly miscible with the 

 fluid surfaces of the cells. Such is the case in the obviously 

 accurate figure which Ilauber gives (Fig. 117, C) of the third mode 

 of conjunction in the four-celled stage of the frog's egg. Here 

 Rauber is most careful to point out that the furrows do not simply 

 "cross," or meet in a point, but are separated by a little space, 

 which he calls the Polgrubchen, and asserts to be constantly present 

 whensoever the polar furrow, or Brechungslinie, is not to be 

 discerned. This little interposed space, with its contained drop 

 of fluid, materially alters the case, and implies a new condition 

 of theoretical and actual equihbrium. For, on the one hand, we 

 see that now the four intercellular partitions do not meet ove 

 another at all ; but really impinge upon four new and separate 

 partitions, which constitute interfacial contacts, not between cell 

 and cell, but between the respective cells and the intercalated 

 drop. And secondly, the angles at which these four little surfaces 

 will meet the four cell-partitions, will be determined, in the usual 

 way, by the balance between the respective tensions of these several 

 surfaces. In an extreme case (as in some pollen-grains) it may be 

 found that the cells under the observed circumstances are not truly 

 in surface contact : that they are so many drops which touch but 

 do not "wet" one another, and which are merely held together 

 by the pressure of the surrounding envelope. But even supposing, 



* Precisely the same remark was made long ago by Driesch: "Das so oft 

 schematisch gezeichnete VierzeUenstadium mit zwei sich in zwei Punkten scheidende 

 Medianen kann man wohl getrost aus der Reihe des Existierenden streichen," 

 Entw. mech. Studien, Z. f. w. Z. Lni, p. 106, 1892. Cf. also his Math, mechanische 

 Bedeutung morphologischer Probleme der Biologie, Jena, 59 pp. 1891. 



