THB RABBIT. 43 



duced is by the growth of pre-existing protoplasm through 

 assimilation of food-material. The only way in which a 

 new cell is known to be produced is by the division of a 

 pre-existing cell, or (more rarely) the fusion of two pre- 

 existing cells into one; and in all such processes of cell- 

 division and fusion the nucleus plays an important part. 

 The only ways in which a new nucleus is ever known to 

 be produced are the division of a pre-existing nucleus, and 

 (more rarely) the fusion of two pre-existing nuclei. 



8. Structure of Protoplasm. All protoplasm, when examined 

 under the highest powers of the microscope, is found to consist of 

 (1) a rather less fluid portion, the spongioplasm, arranged in a 

 complicated network; (2) a more fluid portion, the hyaloplasm, 

 which occupies the interstices of the spongioplasm just as water 

 may fill the interstices of a common sponge ; (3) granules of various 

 kinds, in the hyaloplasm. From the facts that in some cells the 

 granules diminish in number when the cell is starved, while in 

 secreting cells they appear in great abundance when the secretion 

 is about to be formed, it seems evident that the granules are not 

 really part of the protoplasm, but either anastases, which are on 

 the way to become protoplasm, or katastases, which have been 

 formed by the decomposition of the protoplasm. 



9. Structure of the Nucleus. The nucleus* under ordinary 

 conditions is surrounded by an exceedingly delicate film the 

 nuclear membrane (fig. 7, A). The substance within this resembles the 

 protoplasm outside, in consisting of a less fluid network with more 

 fluid substance in its interstices ; but the threads of the network 

 are thicker, and they stain deeply when treated with carmine or 

 other coloured reagents. Certain rounded bodies lying within the 

 nucleus also stain deeply : these are called nucleoli. The difference 

 in the action of staining reagents on the several parts proves a 

 chemical difference between them : it is therefore convenient to 

 speak of the meshwork and the nucleoli collectively as the chromatin 

 of the nucleus ; while the nuclear membrane and more fluid central 

 mass is called the achromatin. 



10. The Centrosphere is a minute body stated to be always 

 present in a cell just outside the nuclear membrane. It is especially 

 conspicuous during the processes of division of the nucleus and 

 cell, and there is some doubt as to whether it really exists at other 

 times. The innermost portion of the centrosphere is distinguished 

 as the centrosome. 



11. Karyokinesis. Division of the nucleus always precedes 

 division of the cell: the separation from the cell of part of the 

 protoplasm without a nucleus is not true cell-division, and the 



