THE CONTRACTILE VACUOLE 405 



to deal first with those pertaining to origin of the vacuole — to see, if 

 possible, just where this organelle comes from, if it does not exist in 

 the cell as a permanent structure. Then, having traced its origin, questions 

 dealing with structure and function will follow in more logical order. 

 An attempt will be made to follow this general plan, but the very nature 

 of the subject will necessitate digressions from time to time. 



While a conscientious review of the literature has been attempted, it 

 is quite possible that important publications have been overlooked. It is 

 hoped that this will prove not to be true, not only for the sake of com- 

 pleteness, but also for the sake of giving credit where credit is due. The 

 author herewith offers his apologies to anyone whose labors have not 

 been acknowledged. 



The Origin of Contractile Vacuoles 



Metcalf (1910) noticed in amoeba of the proteus type that the vacu- 

 ole is surrounded by a layer of granules of the same approximate size 

 and appearance as the "microsomes" of the general cytoplasm. When 

 the vacuole is of moderate size, these granules form a layer on its surface 

 one granule thick; when the vacuole is fully distended, as just before 

 systole, there are spaces between the granules; but when the vacuole is 

 small the layer may be several granules thick. At systole the vacuole 

 usually collapses completely, and the granules may be seen clumped to- 

 gether in the region of the cytoplasm previously occupied by the vacuole. 

 The new vacuole arises in the midst of these granules, and is formed by 

 the fusion of several small vacuoles. According to Metcalf, who reported 

 observations which sometimes lasted for as long as several hours on a 

 single organism, the vacuole never arises in any other part of the body 

 under normal conditions, except among the granules which surrounded 

 it before its last contraction. From these observations he concludes that 

 the granules are associated in some way with the origin and the function 

 of the vacuole, and for this reason calls them "excretory granules." How- 

 ever, he decides that the granules are not essential for life, since most 

 of them, together with the vacuole, may be removed from an Amoeba 

 by operation without a fatal result. Under these conditions a new vacuole 

 develops, although there are few if any granules to be seen surrounding 

 it when it first appears. Metcalf reaffirmed his statement concerning these 

 observations in 1926. 



