VII] OF THE POLAR FURROW 311 



and their corresponding partitions. In this case, the polar 

 furrow is seen to be sinuously curved, and Rauber tells us that 

 its curvature gradually alters : as a matter of fact, it (or rather 

 the partition-wall corresponding to it) is gradually setting itself 

 into a position of equilibrium, that is to say of equiangular contact 

 with its neighbours, which position of equihbrium is already 

 attained or nearly so in Fig. 117, A. In Fig. 117, C, we have a 

 very different condition, with which we shall deal in a moment. 

 According to the relative magnitude of the bodies in contact, 

 this "polar furrow" may be longer or shorter, and it may be so 

 minute as to be not easily discernible ; but it is quite certain that 

 no simple and homogeneous system of fluid films such as we 

 are deahng with is in equilibrium without its presence. In the 

 accounts given, however, by embryologists of the segmentation of 

 the egg. while the polar furrow is depicted in the great majority 



A B C 



Fig. 117. Various ways in which the four cells are co-arranged in 

 the four-celled stage of the frog's egg. (After Rauber.) 



of cases, there are others in which it has not been seen and some 

 in which its absence is definitely asserted*. The cases where four 

 cells, lying in one plane, meet in a point, such as were frequently 

 figured by the older embryologists, are very difficult to verify, 

 and I have not come across a single clear case in recent literature. 

 Considering the physical stability of the other arrangement, 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 am much inclined to attribute to 



* Thus Wilson (J. of Morph. vni, 1895) declared that in Amphioxus the polar 

 furrow was occasionally absent, and Driesch took occasion to criticise and to throw 

 doubt upon the statement {ArcJi. f. Entw. Mech. i, 1895, j). 418). 



