76 SURVEY OF THE REPRODUCTIVE PROCESS 



greater extent than in the vegetable cell, whose motile 

 powers are consequently less marked and less constant. 



2. The predominance of albuminous compounds over 

 those ternary principles, such as cellulose, starch, and chlo- 

 rophyll, which form the bulk of the vegetable cell and its 

 contents, and the absence of the green colour which the 

 action of light on these vegetable principles evolves. 



The Protozoa, on the elimination of these spurious or 

 intruded species, constitute a truly natural group, but one, 

 at the same time, hardly admitting of any good general 

 definition, applicable to the several forms it includes, being 

 distinguished from those referable to the other primary 

 divisions of the Animal Kingdom chiefly by negative cha- 

 racters, such as the absence of a nervous system and of 

 organs of sense, and in many even of a distinct alimentary 

 apparatus. They are sometimes described as unicellular, 

 ■svith a nucleus or minute solid particle, and certain clear 

 sj)aces or vacuoles in their interior,* but it is to be observed 

 that no true cell-wall is developed in the lowest forms — the 

 body consisting of a mere mass of plastic jelly, without any 

 distinct membrane bounding its exterior. 



In the greater number of the Protozoa, reproduction is 

 only as yet known to take place in the way of monogenesis. 

 Two leading modifications of this are admitted, though they 

 are not alw^ays to be distinguished from each other — -Jissioti 

 and ciem7nation.'\ Fission, or the spontaneous separation 

 of the body into two or more segments, prevails mostly in 

 the higher group known as Infusorial Animalcules — gem- 



* Greene's Manual of the Protozoa, p. 2. 

 f Fission, as Professor Owen remarks (Parthenogenesis, p. 10), though 

 it presents a wide prima facie diversity from ordinary gemmation, in half 

 of the body of the parent, instead of only a small portion of it, going to 

 form that of the offspring, is after all only a modification of the other 

 process, the difference depending on the very small size in gemmation of 

 the portion of parenchjnna w4iich takes on the new development, in rela- 

 tion to the whole body, and on its superficial position. 



