1891.] MICEOSCOPICAL JOURNAL. 173 



Introduction to Elementary Biology. 



By henry L. OSBORN, 



HAMLTNB, MINN. 



Part III. — Cell Reproduction. 



{Continued from page /JJ.) 



1. General Statement. — Cells not only have the powers which we 

 have spoken of. but they can, under certain conditions, by various 

 modes, divide into two cells, each retaining the powers and attaining 

 by grow'th the full size normal to that kind of cell. Thus the cells 

 increase in number. The yeast cell, for instance, developes a small 

 " bud," which rapidly enlarges into a second cell and produces a third, 

 meanwhile the first may have produced another. Colonies of yeasts 

 thus related are the common thing in actively growing yeast. Some 

 cells increase by " fission," that is, by splitting into two equal halves, 

 each one of which at once enlarges to be the normal size, thus doubling 

 the bulk of the cell product. Cells, in the growing tips of any root, 

 as for instance the onion roots, increase very rapidly in this manner. 



2. Details of Reproduction by Fission. — It is now very well 

 ascertained that the nucleus presides over the process of reproduction 

 by fission. In growing stuctures, as for instance the developing eggs 

 of animals or the rapidly growing tissue of plants, cells can be seen in 

 which at a certain stage the nuclear substance is collected into two sep- 

 arate portions which are no longer round as at first, but in the form of 

 long, uneven finger-like processes starting from a central portion. 

 Such. cells are in the first stage of fission. Later, the cell-wall grows 

 down between these nuclei and the protoplasm separates into two 

 portions, the cell walls meet and join, and two cells are produced. 

 This process can be seen in Atnoeba and in many simple animals, as 

 well as in the growth of eggs of the more complicated higher animals. 



3. Results of Reproduction by Fission. — Fission may split a 

 cell into two halves, which soon separate and have no further interest in 

 each other. Protococcus, the green plant on bark of trees, illustrates 

 this. Here very often the cells remain in contact for a time, but are 

 virtually independent bodies, and the plant for this reason is called uni- 

 cellular or of one cell, and the fission is called " discontinuous fission." 

 In other cases, the cells may remain connected after fission and •' continu- 

 ous fission." Various forms compounded of cells or " multicellular" 

 may result. Nostoc illustrates this where the cells regularly form 

 chains of slightly connected spherical cells. Also SpirogyrUs when 

 the cells are cylindrical and being placed end to end form long threads 

 or "filaments," and O^czV/ar/a, where the cells have motile function. 

 Among animals, many cases occur of unicellular ones, or animals 

 loosely compounded into what are called colonies, as for instance, Vor- 

 ticella and Codosiga. and there are some multicellular, as for instance, 

 the Dicyemids^ which are little more than strings of cells ; but the mul- 

 ticellular animals as a rule are a long ways more complex than the 

 simplest multicellular plants like •Spirogyra and Oscillaria. Before 

 considering the results of continuous fission, let us note some of the 

 conditions upon which cell-reproduction depend. 



4. Conditions of Cell Reproduction. — Yeast serves very well for 



