PINOCYTOSIS 165 



Sieke\itz's diagram (Fig. 5) and as discussed in Dr. Porter's review, a 

 similar process is generally assumed to occur in the discharge of secretion 

 products formed in the endoplasmic reticulum. It would seem that the 

 endoplasmic reticulum, if it reallv can be shown to torm the link between 

 the ingestion and secretion pathways of the cell, might provide a mechanism 

 that could explain the selectivity of pinocytosis. 



With regard to this problem of selecti\ity we are still very much in the 

 dark. It is true that the process of surface adsorption prior to pinocytosis 

 affords a specific enrichment of the adsorbed solute in the pinocytosis 

 vacuoles. But this can cover only part of the problem. So far no one has 

 been able to give a satisfactory answer to the question : What happens to 



v-.^.. 



f-€5vti).' 



Fig. 6. Defaecation in amoebae. Two instances of defaecation. Above: food 

 remnants; below: cluster of crystals. In both instances the vacuole is seen at the 

 right, brought close to the surface in the tip of a short pseudopod. At the left, the 

 objects have been expelled by rupture of the membrane and lie free in the medium. 



(After Andresen and Holter [i].) 



substances which are dissohed in the fluid ingested together with the 

 surface membrane and which are of no use, or even toxic, to the cell .'* 

 There must be some opportunity for a highly specific process of discrimina- 

 tion somewhere, but it has not seemed very satisfactory to explain any 

 high degree of discrimination on the basis of mere changes in the permea- 

 bility of the vacuolar membrane. For this it seems necessary to assume the 

 action of some sort of active transport mechanism in the interior of the 

 cell, situated either at the membranes of the pinocytosis vesicles, or 

 perhaps in the membrane system of the endoplasmic reticulum. Again the 

 experimental evidence at the present time does not justify more than very 

 tentative speculation on this important point. 



