6o2 WILSON. [Vol. VIII. 



14) by showing that the axis of the spindle coincides with the 

 direction of greatest elongation in the surrounding protoplas- 

 mic mass ; from which it follows at once that the first three 

 cleavages of a spherical mass will be at right angles to one 

 another and successively in the three dimensions of space, as 

 in the radial type. Pfliiger (21), and especially Driesch (8), 

 have shown finally that the direction of protoplasmic elonga- 

 tion (and consequently that of the spindle-axis) may be deter- 

 mined by pressure, but it does not yet appear whether this is 

 a general law. Roux's contradictory results on frog's eggs 

 remain unexplained ; and there are cases {e.g., in the cambium- 

 cells of plant-stems) in which the plane of division is regularly 

 parallel to the long axis of the cell and at right angles to the 

 direction of pressure. Still, the fact remains that the radial 

 type of cleavage is determined by causes, whether physical or 

 physiological, that concern the embryonic stages alone, and 

 have no direct connection with the adult state. 



With the bilateral type the case is widely different, for the 

 cleavage is more or less completely dominated by the sym- 

 metry of the adult body, which may be manifested from the 

 very beginning. It does not seem possible, in the present 

 state of our knowledge, to explain this bilaterality as the result 

 of purely mechanical conditions such as pressure, distribution 

 of yolk, and the like. It occurs in small ova with total cleav- 

 age (tunicate) as well as in large yolk-laden eggs with partial 

 cleavage (cephalopod) ; it is in some cases actually opposed to 

 mechanical conditions. In Clavelina, for example, as may 

 clearly be seen from Van Benedin's and Julin's figures {e.g., 

 Figs. 7, 8, 10, etc.), the bilateral arrangement of the blasto- 

 meres causes the cell-group to depart widely from the arrange- 

 ment demanded by the law of minimal surfaces. Thus, along 

 the median line four cleavage-planes generally meet at a point, 

 instead of three as the law requires ; and this is only gradually 

 corrected by displacements of the blastomeres as the cleavage 

 advances. The same is true of the early stages of the frog 

 {cf. Rauber, No. 23, Figs. 34, 35). It seems necessary to con- 

 clude, therefore, that bilaterality in cleavage is an inJierited 

 character that has arisen by the remodelling of a non-bilateral 



