THE CONTRACTILE VACUOLE 443 



Secretion is in essence a phenomenon of "granule" or droplet formation. 

 Starting with a single such secretory droplet about to be expelled from the 

 cell, we find it possible to trace its origin step by step to a minute vacuole, 

 which has thus from the beginning served as a segregation center for a 

 specific secretion-material. The primordial vacuole is found to arise in that 

 zone of the cell characterized by the presence of the Golgi apparatus, and 

 the evidence indicates, if it does not demonstrate, that the primordial 

 vacuole arises through the activity of the Golgi substance and undergoes a 

 part at least of its development in contact with, or imbedded in, the Golgi ap- 

 paratus. 



The idea of Nassonov, as developed in 1925 and 1926, as well as that 

 of MacLennan (1933), concerning the origin of protozoan vacuoles 

 could hardly be expressed more exactly. 



The outstanding features of contractile vacuoles, taken collectively, 

 then, do not lie in differences among them, but rather in similarities, 

 both morphological and physiological. Another fundamental link in the 

 kinship between all cells seems to be established by the apparent homol- 

 ogy, both structural and functional, between the protozoan and the meta- 

 zoan Golgi apparatus. 



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