STRUCTURAL BASIS OF THE HUMAN BODY. 11 



difference between the protoplasm of the cell and nuclei, as the former is 

 destroyed by these reagents. 



(2. ) Their quality of staining in solutions of carmine, haematoxylin, 

 etc. Nuclei 'are most commonly oval or round, and do not generally 

 conform to the diverse shapes of the cells; they are altogether less varia- 

 ble elements than cells, even in regard to . size, of which fact one may see 

 a good example in the uniformity of the nuclei in cells so multiform as 

 those of epithelium. But sometimes nuclei appear to occupy the whole 

 of the cell, as is the case in the lymph corpuscles of lymphatic glands 

 and in some small nerve cells. 



Tlieir position in the cell is very variable. In many cells, especially 

 where active growth is progressing, two or more nuclei are present. 



The nuclei of many cells have been shown to contain a fine intra- 

 nuclear network in every respect similar to that described above as intra- 

 cellular (Fig. 3), the interstices of which are occupied by semi-fluid pro- 

 toplasm. 



III. Reproduction. The life of individual cells is probably very 

 short in comparison with that of the organism they compose: and their 

 constant decay and death necessitate constant reproduction. The mode 

 in which this takes place has long been the subject of great controversy. 



In the' case of plants, all of whose tissues are either cellular or com- 

 posed of cells which are modified or have coalesced in various ways, the 

 theory that all new cells are derived from pre-existing ones was early ad- 

 vanced and very generally accepted. But in the case of animal tissues 

 Scliwann and others maintained a theory of spontaneous or free cell for- 

 mation. 



According to this view a minute corpuscle (the future nucleolus) 

 springs up spontaneously in a structureless substance (blaslbma) very much 

 as a crystal is formed in a solution. This nucleolus attracts the surround- 

 ing molecules of matter to form the nucleus, and by a repetition of the 

 process the substance and wall are produced. 



This theory, once almost universally current, was first disputed and 

 finally overthrown by Remak and Virchow, whose researches established 

 the truth expressed in the words "Omnis celiula e cellula." 



It will be seen that this view is in strict accordance with the truth 

 established much earlier in Vegetable Histology that every cell is de- 

 scended from some pre-existing (mother-) cell. This derivation of cells 

 from cells takes place by (1) gemmation, or (2) fission or division. 



(1.) Gemmation. This method has not been observed in the human 

 body or the higher animals, and therefore requires but a passing notice. 

 It consists essentially in the budding off and separating of a portion of 

 the parent cell. 



(2.) Fixxion or Division. As examples of reproduction by fission, we 

 may select the ovum, the blood coll, and cartilage cells. 



