No. I.] THE EMBRYOLOGY OF CREPIDULA. 187 



lose their rounded outlines ; they are pressed more and more 

 closely together, rotations occur, and consequently pressure 

 surfaces are developed, and we have the principle of surface 

 tension and, perhaps, of mutual attraction between the cells 

 {Cytotropis7}ius, Roux '94) asserting itself in the appearance of 

 spiral cleavages. 



(2) Significance of Spiral Cleavages. — So far as the mere 

 rotation of blastomeres and the consequent formation of polar 

 furrows and pressure surfaces is concerned, I quite agree with 

 Wilson that " the spiral type owes its peculiarities entirely to 

 mechanical conditions, the blastomeres assuming the position 

 of greatest economy of space, precisely like soap bubbles or 

 other elastic bodies." This form of cleavage alone fulfills the 

 conditions of minimal contact surfaces, and considered from 

 the purely physical standpoint, it is a wonder that it should 

 ever fail to occur. Spiral cleavages, then, in general, are cer- 

 tainly due to the general physical phenomenon of surface ten- 

 sion ; but the fact that they occur in definite directions is just 

 as certainly due to something else. The absolute constancy 

 of direction in certain cases of spiral cleavage is a thing which 

 no merely extrinsic factors can possibly account for. The 

 alternate directions of the spirals is but an expression of 

 alternation in general, and each successive cleavage finds the 

 sufficient cause of its direction in the direction of the preced- 

 ing one until we reach the first cleavage. Why is the first 

 division dexiotropic rather than laeotropic .-• I cannot at pres- 

 ent answer this question, but it is obvious that the cause of 

 this constancy of direction must be intrinsic rather than ex- 

 trinsic, and that it must be sought for, not in the mechanical 

 conditions of surface tension, but rather in the structure of 

 the unsegmented egg itself. 



The direction of the spirals has presumably a profound 

 influence upon the entire development. Associated with it 

 is the formation of the mesoblast and the greater part of the 

 adult body from the left posterior macromere in cases where 

 the spirals are not reversed, whereas it is the right posterior 

 macromere which gives rise to these structures in cases of 

 reversed cleavage, as Crampton ('94) has shown in the case of 



