162 SOME RECENT DEVELOPMENTS 



quickly arose, which the progress of knowledge has 

 served rather to widen than to restrict. Schwann's 

 idea of a cell was " a vesicle closed by a solid mem- 

 brane, containing a liquid in which floats a nucleus 

 enclosing a nucleolus, and in which also one may 

 find small granular bodies." This definition was 

 soon challenged. The cell wall was first attacked, 

 and shown by Nageli in 1845 to be not essential 

 to a cell. Others speedily confirmed Nageli on this 

 point, and in 1857 we find Leydig defining a cell 

 as " a soft substance containing a nucleus." Next 

 the nucleus received attention. Max Schultze, one 

 of the most careful of observers, denied its existence 

 in Amceba porrecta ; other investigators quickly 

 followed on the same line, and by Haeckel a dis- 

 tinct sub-kingdom, the Protista, was proposed for 

 the reception of forms such as Protamceba, Proto- 

 myxa, and Protomonas, which were supposed to be 

 devoid of nucleus and were regarded as possibly 

 representing the parent forms from which animals 

 and plants have alike descended. Further research 

 has however tended to check rather than to con- 

 firm these statements; and the use of special re- 

 agents and more refined histological methods has 

 shown that nuclei are present in many forms, such 

 as the Foraminifera, in which their existence was 

 at first denied. 



The more recent investigations have also shown 

 that the nucleus is not as was at first supposed a 

 structure always presenting the same characters, 

 but that it may vary greatly in form, structure, and 

 relations, even amongst the simplest animals, or 



