4 ZOOLOGY FOR MEDICAL STUDENTS . CHAP. 



happens is that the ectoplasm ruptures and a quantity of endoplasm 

 wells out at the perforation. This extrusion forms then an extension 

 of the body of the Amoeba composed at first entirely of endoplasm (Fig. 2, 

 A). Very soon however the superficial layer loses its granularity, be- 

 comes highly refracting and assumes the characters of typical ectoplasm. 

 Portions of the original ectoplasm covered in and sheltered from contact 

 with the water by the newly formed lobopod gradually fade away, losing 

 the characters of ectoplasm and taking on those of endoplasm (Fig. 2, B). 

 We are taught by such observations as these that ectoplasm and endo- 

 plasm are not fundamentally distinct substances, and that the characters 

 which normally differentiate ectoplasm from endoplasm are merely 

 temporary modifications taken on by the surface layer of the cytoplasm 

 as a reaction to its contact with the surrounding water. 



The pseudopodia are not permanent organs : they are pushed out 



A B 



FIG. 2. 

 Illustrating the formation of an eruptive lobopod. 



as has been described and they may be drawn in, apparently by the 

 shrinkage of their containing layer of ectoplasm. Although they may 

 be pushed out from any part of the surface indifferently, their formation 

 is as a rule at any one moment of time taking place more actively on 

 one particular portion of the surface. The result is that, as the pseudo- 

 podia are pushed out more actively in one particular direction, the 

 Amoeba undergoes change of position towards that direction, pseudopodia 

 pointing in other directions shrinking in and disappearing. There are 

 few sights more impressive to the eye that has a brain behind it than 

 that of a large healthy Amoeba with its protoplasm actively streaming 

 out into its pseudopodia, carrying on that old world type of movement, 

 just as it in all probability took place at the dawn of evolution when 

 living substance first began to move. 



Although the Amoeba can push out its pseudopodia when floating 

 freely suspended in the water it can creep along as a whole only upon a 



