CONJUGATE. 



41 



it lives in the intestines of those attacked by the disease, and 

 gives off a sti*ong poison which enters the body. It is easily 

 ciiltivated as a saprophyte. 



A great many circumstances seem to show that a number of 

 other infectious diseases (syphilis, small-pox, scarlet-fever, measles, 

 yellow-fever, etc.) owe their origen to parasitic Bacteria, but this 

 has not been proved with certainty in all cases. 



It has been possible by means of special cultivations (ample 

 supply of oxygen, high temperature, antiseptic materials) to pro- 

 duce from the parasitic Bacteria described above (e.g. the fowl-cholera 

 and the anthrax Bacteria) physiological varieties which are distinct 

 from those appearing in nature and possess a less degree of 

 " virulence," i.e. produce fever and less dangerous symptoms in 

 those animals which are inoculated with them. The production 

 of such physiological varieties has come to be of great practical 

 importance from the fact that they are used as vaccines, i.e. these 

 harmless species produce in the 

 animals inoculated with them 

 immunity from the malignant in- 

 fectious Bacteria from which they 

 were derived. This immunity is 

 effected by the change of the 

 products of one or more of the 

 Bacteria, but we do not yet know 

 anything about the way in which 

 they act on the animal organism. 

 The white blood corpuscles, ac- 

 cording to the Metschnikoff, play 

 the part of " Phagocytes " by 

 absorbing and destroying the less virulent Bacteria which have 

 entered the blood, and by so doing they are gradually enabled to 

 overcome those of a more virulent nature. 



Class 5. Conjugatae. 



The Algae belonging to this class have chlorophyll, and pyre- 

 noids round which starch is formed. The cells divide only in one 

 direction, they live solitarily, or united to form filaments which 

 generally float freely (seldom attached). Swarm-cells are want- 

 ing 1 . The fertilisation is isogamous (conjugation) and takes place by 

 means of apJanogametes. The zygote, after a period of rest, 

 produces, immediately on germination, one or more new vegetative 



FIG. 31. a and i> The same blood-cell 

 of a Frog: a in the act of engulfing 

 an anthrax-bacillus ; b after an interval 

 of a few minutes when the bacillus has 

 been ab? orbed. 



