CHAPTER I. -THE CELL. I 2 7 



The protoplasm is a nearly transparent, semi-fluid substance, 

 of very complex character. Its chemical formula has not been 

 determined, but it is known to contain carbon, hydrogen, oxygen, 

 nitrogen, sulphur, and some forms of it, at least, phosphorus. It 

 is probably a combination of several different albuminous sub- 

 stances, with admixtures in larger or smaller proportions of 

 water, carbo hydrates, oils, and mineral matters. The outer 

 layer of the protoplasm is in intimate contact with the cell-wall, 

 and, being somewhat tougher and firmer than the rest, consti- 

 tutes a kind of inner cell-wall called the primordial utricle. This 

 is imperfectly differentiated into two layers, an outer one, called 

 the ectoplasm, which is perfectly transparent and free from gran- 

 ules, Fig. 361, c, and an inner one, called the endoplasm, which 

 contains minute granules,. and often, also, chlorophyll corpus- 

 cles. Fig. 361, d. Located somewhere in the interior of the cell, 

 or else in contact with the endoplasm, is a rounded, granular 

 and more highly refractive body called the nucleus, Fig. 361,/! 

 and in the interior of the latter, one or more opaque bodies 

 called the nucleoli. In a fully formed and active cell, like that 

 represented in the figure, there are threads or bands of proto- 

 plasm, connecting the nucleus with the primordial utricle, and 

 in these bands chlorophyll-corpuscles or other proteid masses 

 commonly occur, Fig. 361, i. The spaces between them contain 

 cell-sap, and are called vacuoles. Fig. 361, e. The position of 

 the nucleus and the connecting bands and threads of protoplasm 

 is not constant, but continually undergoes change. The move- 

 ments are ordinarily slow, but they are sometimes sufficiently 

 rapid to be directly observable by means of the microscope. 

 Currents are also observed about the nucleus, in the endoplasm, 

 and in the threads and bands, showing that the living matter of 

 the cell is in a state of constant activity. 



In the very young cell the wall is exceedingly thin, and 

 apparently homogeneous, the vacuoles are absent, and the entire 

 area inclosed by the wall appears to be filled with protoplasm, 

 Fig. 362 ; but as the cell grows older, its walls become thickei 

 and differentiated as described above ; they often be'come ex 

 cessively thickened, and not infrequently undergo important 

 chemical changes ; by the expansion of the cell-wall, also, the 

 cavity of the cell increases faster than the contained protoplasm ; 



