AN ARTIFICIAL OSMOTIC CELL 41 



tact with the surrounding medium. The process of growth 

 proper now begins. 



The opening in the sac, which is at first irregular, rapidly 

 becomes round. It is then seen to be raised upon a protuber- 

 ance which turns from a translucent red-bro^^^l (the color of 

 the osmotic sac) to an opaque black. This protuberance be- 

 comes elongated into a stem whose thickness is uniform except 

 at the end, where it tapers into a tall cone, open at the apex. 

 This conical end of the stem retains throughout growth the trans- 

 lucency and red-brown color of the original protuberance. The 

 entire process, from the moment the crystaj is dropped into the 

 medium until the formation of a perfect stem, occupies (at room 

 temperatVu'e) about one minute. 



The stem consists of an outer, a middle and an inner layer. 

 During the process of growth no sharp hne of demarcation can 

 be seen between the sohd tip of the stem and the solution in 

 which it is bathed (see fig. 1, B). The first discoverable struc- 

 ture is at the upper edge of the inner surface of the stem and 

 consists of minute refractive colorless sphericaJ bodies which 

 become irregular as they coalesce to form a continuous surface. 

 This structure, which constitutes the thin lining of the lumen of 

 the stem, may be observed in prepared microscopic specimens 

 as a yellow band passing along the entire length of the stem. 

 Where the refractive bodies have formed a continuous surface, 

 this surface becomes covered externally with a translucent red- 

 brown coating, which constitutes the thin outer covering of 

 the stem. When fully developed this layer appears perfectly 

 homogeneous. During the process of growth, however, it can 

 be seen to be formed by the conglomeration of a vast number of 

 minute spherical bodies of a red-brown color. Between these 

 two layers a third, thin black layer, now makes its appearance, 

 which thickens, until at the junction of the cone-shaped tip 

 and the body of. the stem, it outstrips in thickness both the in- 

 ner and outer layers. The formation of the middle layer after 

 the formation of the outer and inner ones accounts for a state 

 of tension which exists in the outer layer. This tension is mani- 

 fest from the fact that when a stem is removed from the solu- 



