GENERAL BIOLOGY. 9 



ular tissues, besides those constituting its blood-vessels, lym- 

 phatics, and nerves. But all of the cells of each tissue have, 

 speaking generally, the same function. The student is referred 

 to works on general anatomy and histology for classifications 

 and descriptions of the tissues. See also page 603. 



The statements of this chapter will find illustration in the 

 pages immediately following, after which we shall return to 

 the subject of the cell afresh. 



Summary. — The typical cell consists of a wall,. protoplasmic 

 contents, and a nucleus. The vegetable cell has a limiting 

 membrane of cellulose. Cells undergo differentiation and may 

 be united into groups forming tissues which serve one or more 

 definite purposes. 



The chemical constitution of protoplasm is highly complex 

 and unstable. The nucleus plays a prominent part in the life- 

 history of the cell, and seems to be essential to its perfect devel- 

 opment and greatest physiological eflBciency. 



UNICELLULAR PLANTS. 



Yeast (Torula, Saccharomyces Cerevisioe). 



The essential part of the common substance, yeast, may be 

 studied to advantage, as it affords a simple type of a vast group 

 of organisms of profound interest to the student of physiology 

 and medicine. To state, first, the main facts as ascertained by 

 observation and experiment : 



Morphological. — ^The particles of which yeast is composed 

 are cells of a circular or oval form, of an average diameter of 

 about ^-jnnr oi an inch. 



Each individual torula cell consists of a transparent homo- 

 geneous covering (cellulose) and granular semifluid contents 

 (protoplasm). Within the latter there may be a space (vacu- 

 ole) filled with more fluid contents. 



The various cells produced by budding may remain united 

 like strings of beads. Collections of masses composed of four 

 or more subdivisions (ascospores), which finally separate by rup- 

 ture of the original cell wall, having thus become themselves in- 

 dependent cells, maybe seen more rarely (endogenous division). 



The yeast-cell is now believed to possess a nucleus. 



Chemical. — When yeast is burned and the ashes analyzed, 

 they are found to consist chiefly of salts of potassium, calcium, 

 and magnesium. 



