OF PROTOZOA. 287 



to be the reverse of that of Podophrya (fig. 25). After under- 

 going a peculiar kind of segmentation, the egg is changed to a 

 simple globule, which, for want of a distinguishing feature, 

 might remain unnoticed, were it not associated with numerous 

 others of the same kind in the reproductive organ. In process 

 of time this globule becomes endowed with a clear, hollow 

 space, which, by its regularly alternating contraction and expan- 

 sion, may be recognized as a contractile vesicle. This is the 

 earliest condition in which the embryo can be distinguished, as 

 such, in an isolated state. Presently there is added to this a 

 covering of cilia, of two kinds ; the one, however, usually pre- 

 ceding the other, although with some irregularity. The one kind 

 are simple globe -tipped threads (fig. 188, /, /) 

 which are very few in number, and do not vibrate ; 

 the other kind are like ordinary vibrating cilia, and I 



are numerously distributed over the whole body. 

 At the period of birth sometimes only the globe- 

 tipped threads (/,/) are present, and the vibrating 

 cilia appear afterwards. The young Paramecium Fig. 

 represented here (fig. 188) possesses both kinds of cilia, and, 

 moreover, exhibits a trace of the characteristic oblique mouth 

 (m) at one side of its otherwise symmetrical body. To com- 

 plete its growth it needs to drop the globe-tipped cilia, develop 

 another contractile vesicle near the posterior end, perfect the 

 organization of its, as yet, incipient digestive system, and in- 

 crease its size and proportions to a considerable extent. It has 

 already developed far enough to enable one to recognize in its 

 features the Protozoan type ; but it does not possess such defi- 

 nite characters as would distinguish it from those other kinds 

 of infusorians which are closely allied to the one from which it 

 originated. It is not always an easy matter to distinguish one 

 kind of adult Paramecium from another, and much less readily 

 can it be done with the younger. The only means that we 



Fig. 188. The embryo of Paramecium Bursaria. Focke. 350 diarn. m, 

 the first trace of the mouth; cv, the contractile vesicle;/,/, the globe-tipped 

 cilia. From Claparede, 



