COMPARED WITH THOSE OP ANIMALS. 53 



applied by Swann to the tissues of animals, are now ad- 

 mitted by almost all physiologists, the following abridg- 

 ment of them will be acceptable : — 



When there are the appropriate external conditions, the 

 first visible stage of cell-formation consists in the appear- 

 ance of minute granules which trouble the clear gummy 

 solution in the cells, or the interspaces which surround 

 them, rendering it turbid and opaque. Some of these 

 granules collect together and form a nucleus around which 

 other granules (nucleoli) gather, so that they ultimately 

 acquire a larger size than the rest. These nucleated agglo- 

 merations, called by Schleiden, cytoblasts, become each an 

 active centre around which the mucilaginous fluid of the 

 protoplasm organizes itself. As soon as the cytoblasts are 

 fully grown, a fine transparent vesicle developes on one 

 side of them. This vesicle first appears as the segment of 



Fig. 11. 



Cells of a leek, after Quekett. u, nucleus ; i, nucleolus. 



a sphere, the cytoblast forming its flat side and the walls 

 of the vesicle its convex surface. The vesicle continues to 

 expand until at length the cytoblast from which it ori- 



5* 



