vii] OF THE POLAR FURROW 491 



in one plane meet in a point, such as were frequently figured by the 

 older embryologists, are hard to verify and sometimes not easy to 

 believe. Considering the physical stability of the other arrange- 

 ment, the great preponderance of cases in which it is known to 

 occur, the difficulty of recognising the polar furrow in cases where 

 it is very small and unless it be specially looked for, and the natural 

 tendency of the draughtsman to make an all but symmetrical 

 structure appear wholly so, I was wont to attribute to error or 

 imperfect observation all those cases where the junction-hnes of four 

 cells are represented (after the manner of Fig. 172, A) as a simple 

 cross*. As a matter of fact, the simple cross is no very rare pheno- 

 menon, even in the frog's egg; but it is a transitory one, and 

 unstable. Viscosity and friction may enable it to endure for a 

 while, but the partitions inevitably shift into the stable, three-way, 

 configuration. In such a case, the polar furrow manifests itself 

 slowly and as it were laboriously; but in the more fluid soap-bubble 

 it does so in the twinkling of an eye. 



While a true four-rayed intersection, or simple cross, is theoretic- 

 ally impossible save as a transitory and unstable condition, there 

 is another configuration which may closely simulate it, and which 

 is common enough. There are plenty of faithful representations of 

 segmenting eggs in which, instead of the triple junctions and polar 

 furrow, the four cells (and also their more numerous successors) are 

 represented as rounded off, and separated from one another by an 

 empty space, or by a little drop of extraneous fluid, evidertly not 

 directly miscible with the fluid surface of the cells. Such is the 

 case in the obviously accurate figure which Rauber gives (Fig. 

 175, C) of his 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 Polgrilbchen, and asserts to be 

 constantly present whensoever the polar furrow, or Brechungslinie, 

 is not to be discerned. This little interposed space with its con- 

 tained drop of fluid materially alters the case, and implies a new 



* The same remark was made long atgo by Driesch: "Das so oft schematisch 

 gezeichnete Vierzellenstadium mit zwei sieh in zwei Punkten scheidende Medianen 

 kann man wohl getrost aus der Rcihe des Existierenden streichen" (Entw, mech. 

 Studien. Z. f. w. Z. liii, p. 166, 1892). Cf. also his Math, mechanische Bedeutung 

 morphologischer Probleme der Biologic, Jena, 59 pp., 1891. 



