348 THE FORMS OF TISSUES [ch. 



as the unequal segmentation of the frog's egg. It is true when the 

 dividing cell is homogeneous, and under the influence of symmetrical 

 forces ; but it ceases to be true when the field is no longer dynami- 

 cally symmetrical, for instance, when the parts difier in surface 

 tension or internal pressure. This latter condition, of asymmetry 

 of field, is frequent in segmenting eggs*, and is then equivalent 

 to the principle upon which Balfour laid stress, as leading to 

 "unequal" or to "partial" segmentation of the egg, — viz. the 

 unequal or asymmetrical distribution of protoplasm and of food- 

 yolk. 



The second rule, which also has its exceptions, is true in a 

 large number of cases; and it owes its validity, as we may judge 

 from the illustration of the repeatedly bisected cube, solely to the 

 guiding principle of minimal areas. It is in short subordinate 

 to, and covers certain cases included under, a much more important 

 and fundamental rule, due not to Sachs but to Errera ; that (3) the 

 incipient partition- wall of a dividing cell tends to be such that its 

 area is the least possible by which the given space-content can be 

 enclosed. 



Let us return to the case of our cube, and let us suppose that, 

 instead of bisecting it, we desire to shut off some small portion 

 only of its volume. It is found in the course of experiments upon 

 soap-films, that if we try to bring a partition-film too near to one 

 side of a cubical (or rectangular) space, it becomes unstable ; and 

 is easily shifted to a totally new position, in which it constitutes 

 a curved cylindrical wall, cutting off one corner of the cube. 

 It meets the sides of the cube at right angles (for reasons which we 

 have already considered) ; and, as we may see from the symmetry 



* M. Robert {I. c. p. 305) has compiled a long list of cases among the molluscs 

 and the worms, where the initial segmentation of the egg proceeds by equal or 

 unequal division. The two cases are about equally numerous. But like many 

 other writers, he would ascribe this equaUty or inequahty rather to a provision 

 for the future than to a direct effect of immediate physical causation : " li semble 

 assez probable, comm-3 on I'a dit souvent, que la plus grande taille d'un blastomere 

 est liee a I'importance et au developpement precoce des parties du corps qui doivent 

 en naitre : il y aurait la une sorte de reflet des stades posterieures du developpement 

 sur les premieres phenomenes, ce que M. Ray Lankester appelle precocious segrega- 

 tion. II faut avouer pourtant qu'on est parfois assez embarrasse pour assignor une 

 cause a pareilles differences." 



