ELEMENTARY VITAL PHENOMENA 



237 



pseudopodia that are short and blunt, incised, thick and finger- 

 shaped, slender and thorn-like, straight and radiating, long and 

 thread-like, dichotomously branched, or reticulate. But all these 

 varieties, which are united with one anotherby innumerable trans- 

 itions, are produced in the same manner, namely, by protoplasm 

 streaming out into the medium centrifugally from the central cell- 

 body. In organisms possessing long, filose pseudopodia, such 

 as Foraminifera {e.g., Orhitolites, Fig. 98), the protoplasm must 

 travel a long way from the centre to the tip of the constantly 



Fig. 97. — Pigment-cells from the skin of the frog ; /, extended ; //, slightly contracted ; III, 

 strongly contracted ; IV, wholly contracted ; the clear spot in the centre of the cell -body is 

 the nucleus. 



lengthening process ; in these fine threads the microscope shows the 

 protoplasm with its granules flowing like the water of a slow 

 stream. This extremely fascinating phenomenon constantly charms 

 the observer and has been vividly described by Dujardin ('41), 

 Max Schultze ('54), and Haeckel ('62), as granular or 'protoplasmic 

 streaming. In the retraction of such pseudopodia the proto- 

 plasmic particles must again travel over the same path in the 

 reverse or centripetal direction. In pseudopodia that are extended 

 to a considerable distance and remain extended for a considerable 

 time, two currents, a centrifugal and a centripetal, are always 



