22 MORPHOLOGY AND CULTURE OP MICROORGANISMS 



Vacuoles. — ^There is always in the cytoplasm one (or several) rather 

 bulky vesicle filled supposedly with an aqueous solution of mineral 

 salts called a vacuole. Vacuoles play an important part in the ab- 

 sorption of liquids by the cell. Owing to the mineral salts dissolved 

 in the vacuole-fluid, the concentration of which is ordinarily higher 

 than that of the surrounding medium, the vacuoles become the center 

 of dsmotic forces which consequently cause a part of the ambient 

 liquid to penetrate the cell and determine its turgescence. 



Very curious vacuoles are found in many protozoa, namely, the 

 pulsating vacuoles (Figs. 7, 11). They are small vacuoles which expand 

 and contract rhythmically, and which are considered as excretory and 

 respiratory organs. The water that has entered the cell gathers in this 

 vacuole and is expelled as it contracts. Probably in crossing the body 

 this water yields its oxygen to the cytoplasm in order to charge itself 

 with carbonic acid and the products of metabolism. 



Reserve Products. — The cytoplasm encloses some structures differ- 

 entiable by means of certain stains or chemical reagents as granulations, 

 but which are not constituent elements of cytoplasm; they come 

 from a secretion of the cytoplasni, and only under certain conditions. 

 These grains may be found either in the cytoplasmic substance itself, 

 or in the vacuoles included in the cytoplasm. Most of these granules 

 are reserve products which appear when nutrition is deficient. Among 

 the reserve products most common in microorganisms are the granules 

 called metachromatic corpuscles (Fig. 11, [A). These bodies, which 

 are the object of a special study in connection with molds and yeasts, 

 are made up of a substance the nature of which is still unknown, and 

 are found in nearly all fungi, in most algse and bacteria, and in many 

 protozoa. 



Glycogen and paraglycogen are equally well distributed in micro- 

 organisms (fungi, protozoa). Among algae, glycogen is found only 

 in the Cyanophycece, but it is elsewhere replaced by starch or para- 

 mylum (Fig. 10), common products of chlorophyllic assimilation. 



There are also the protein substances, such as crystalloids of 

 mucorin scattered in the Mucorince, or the globules of fat common 

 in ail cells (Fig. 12, 5). 



Most of these substances seem to result from the activity of the 

 chondrium structure. Recent investigation shows that the meta- 

 chromatic corpuscles have their rise among the mitochondria. It 



