258 THE FORMS OF CELLS [ch. 



over the surface and gradually forms a hanging drop, approxi- 

 mately hemispherical ; but as more liquid is added the drop 

 sinks or rattier grows downwards, still adhering to the surface 

 film ; and the balance of forces between gravity and surface 

 tension results in the unduloid contour, as the increasing weight 

 of the drop tends to stretch it out and finally break it in two. 

 At the moment of rupture, by the way, a tiny droplet is formed 

 in the attenuated neck, such as we described in the normal 

 division of a cylindrical thread (p. 233). 



To pass to a much more highly organised class of animals, we find the 

 unduloid beautifully exemplified in the little flask-shaped shells of certain 

 Pteropod mollusca, e.g. Cuvierina*. Here again the symmetry of the figure 

 would at once lead us to suspect that the creature lived in a position of 

 symmetry to the surrounding forces, as for instance if it floated in the ocean 

 in an erect position, that is to say with its long axis coincident with the direction 

 of gravity; and this we know to be actually the mode of life of the little 

 Pteropod. 



Many species of Lagena are complicated and beautified by a 

 pattern, and some by the superadd! tion to the shell of plane 

 extensions or "wings." These latter give a secondary, bilateral 

 symmetry to the little shell, and are strongly suggestive of a 

 phase or period of growth in which it lay horizontally on the 

 surface, instead of hanging vertically from the surface-film : in 

 which, that is to say, it was a floating and not a hanging 

 drop. The pattern is of two kinds. Sometimes it consists 

 of a sort of fine reticulation, with rounded or more or 

 less hexagonal interspaces : in other cases it is produced by a 

 symmetrical series of ridges or folds, usually longitudinal, on the 

 body of the flask-shaped cell, but occasionally transversely arranged 

 upon the narrow neck. The reticulated and folded patterns we 

 may consider separately. The netted pattern is very similar to the 

 wrinkled surface of a dried pea, or to the more regular wrinkled 

 patterns upon many other seeds and even pollen-grains. If a 

 spherical body after developing a "skin" begin to shrink a little, 

 and if the skin have so far lost its elasticity as to be unable to 

 keep pace with the shrinkage of the inner mass, it will tend to 

 fold or wrinkle ; and if the shrinkage be uniform, and the elasticity 

 .and flexibility of the skin be also uniform, then the amount of 

 * Cf. Boas, Spolia Atlantica, 1886, pi. 6. 



