GENERAL INTRODUCTION. 



consist really of non-nucleated protoplasm, capable not only of growth 

 but reproduction. They may be distinguished as ' cytods ' from the cell 

 to which a nucleus is essential. It is possible however that the elements 

 of the nucleus are in these cases disseminate. Protoplasm, or ' the physical 

 basis of life,' is a substance of complex chemical composition containing 

 Nitrogen, Carbon, Oxygen and Hydrogen, with Sulphur, Phosphorus, 

 Sodium and Potassium. From the physical point of view it is viscid, 

 "of variable refrangibility, more or less doubly refractile, colourless, hyaline 

 in its purest condition. It appears sometimes to be structureless, but 

 as a rule it is more or less vesicular, consisting of a denser substance 

 (mitome) enclosing droplets of a more fluid character (enchylema, para- 

 mitome), and it is endowed with certain physiological properties, the sum 

 of which constitute life. It is contractile, irritable, possessed of auto- 

 matism, able to convert other protoplasm or less complex compounds, 

 sometimes organic only, sometimes only inorganic, into its own substance. 

 And this nutrition not only maintains the status quo> but if over sufficient 

 for that purpose leads first to the storage of superfluous material in 

 the shape of fat, albumen and starchy bodies ; and secondly, causes a 

 positive increase of bulk, with which is connected the power of repro- 

 duction in its most primitive form a division of the mass into two 

 similar parts. But all these powers are exercised at the cost of a chemical 

 transformation or degradation of the protoplasm itself, in part respiratory, 

 i. e. oxydative. The products of this degradation, Carbon dioxide and 

 various nitrogenous compounds, are useless to the organism, and are 

 excreted. The whole of the vital properties enumerated can be exercised 

 only while the protoplasm is saturated with water. One of -the con- 

 sequences of the vital energies of protoplasm is that as a substance it 

 can never be obtained in a chemically pure condition, which is only 

 approached when it is starved. Otherwise it is laden with the products 

 of progressive and regressive metamorphosis. It may be added that the 

 protoplasm of a cell often gives origin by conversion to an external or 

 internal cell-skeleton, the characters of which, both chemical and physical, 

 are extremely variable l . 



The nucleus of a cell is a structure sharply marked off from the 

 protoplasm. In its simplest state it is homogeneous and more or less 



1 See the introductory chapter in Foster's ' Textbook of Physiology,' and on the movements, &c. 

 of protoplasm, Engelmann, ' Die Protoplasma- und Flimmerbewegung,' in Hermann's Handbuch der 

 Physiologic, i. 1879, p. 343 et seqq., the first part of which is translated in Q. J. M. xxiv. 1884. On 

 the structure and physiology of ciliated cells, see also Engelmann, Pfliiger's Archiv fur Physiologic, 

 xxiii. 1880. 



