The Microscope. 293 



as it seems to us, was in the attitude of the child toward Nature, 

 eager to know its facts and the reasons for them, seeking knowledge 

 for the sake of knowing. There is no wonder that the early investi- 

 gators of microscopic life were enthusiastic; they had abundant 

 reason for this directing sentiment, their devotion and patience laid 

 well the foundations of the science of a great branch of animals. 

 Leuwenhoeck, Jablot, Baker, Trembly, Ledermuller, Perty, Muller 

 and Ehrenberg prepared the way for the brilliant discoveries and 

 broad generalizations of a half century just passed by the renowned 

 students of the simplest living beings. The present knowledge of 

 the protozoa does not compare unfavorably with that of any other 

 assemblage of animals, and is advancing as rapidly; this, too, when 

 only comparatively few skilled workers can contribute to this end; 

 and, moreover, these forms are of little or no practical use or import- 

 ance. Even their once supposed intimate relation as a cause to 

 many distressing maladies, has not been confirmed by recent research, 

 but rather disproved, except, perhaps, in the rare instances of cer- 

 tain blood parasites of aquatic animals. There is then, sufficient 

 love of abstract ti-uth, sufficient enthusiasm in bringing to light 

 new facts and endeavoring to answer grave problems of philosophy, 

 to render possible brilliant discoveries, if not a brilliant scientific 

 epoch. To these sentiments is due our knowledge of and interest 

 in the protozoa. 



The animal kingdom is divided into two natural groups or 

 series, the protozoa and the metazoa. The former includes the uni- 

 cellular forms, or those generally regarded as the equivalent of the 

 histological cell; the latter are multicellular; their tissues are com- 

 posed of histogenetic elements or cells, and these are arranged in 

 two sets, viz: the ectoderm, or outside body- wall, and the endo- 

 derm, or lining of the alimentary cavity. These commence existence 

 as a nucleated cell; their subsequent growth and complexity are the 

 result of cell multiplication and modification mingled with the pro- 

 ducts of cell life. The protozoa do not pass beyond the primitive 

 stage, i. e., cell division giving rise to individuals. None are modi- 

 fied for the sake of others, and all perform similar functions and all 

 the essential functions of an animal. Truly, then, we have here the 

 simplest forms of animal existence possible. Whilst the life of the 

 metazoon may be regarded as the resultant of hosts of individuals 

 comprising it, and among which, division of labor is fully carried 

 out, on the other hand, in the protozoon, we see manifested by each 

 individual, only the capabilities of one element. In this case, then, 

 we deal with the absolute elements, and not with resultants. Here 



