Reproduction and Development. 39 



the embryos, if so we may call them, have a whip-like 

 cilium like the monads. 



Very frequently, however, there is, in the protozoa, a 

 further process, which would seem to be intimately 

 associated with fission or the formation of spores, as the 

 case may be. This is known as conjugation. Among 

 monads, for example, two individuals may meet together, 

 conjugate, -and completely fuse the one into the other. A 

 triangular cyst results. After a while, the cyst bursts, and 

 an apparently homogeneous fluid escapes. The highest 

 powers of the microscope fail to disclose in it any germ of 

 life"; and there, at first sight, would seem to be an end of 

 the matter. But wait and watch ; and there will appear 

 in the field of the microscope, suddenly and as if by magic, 

 countless minute points, which prolonged watching shows 

 to be growing. And when they have further grown, each 

 distinct point is seen to be a monad. 



In the slipper-animalcule, conjugation 'is temporary. 

 But during the temporary fusion of the two individuals 

 important changes are said to occur. In these infusorians 

 there is, beside the nucleus, a smaller body, the paranu- 

 cleus. This, in the case of conjugating pararncecia, appears 

 to divide into two portions, of which one is mutually ex- 

 changed. Thus when two slipper-animalcules are in con- 

 jugation, the paranucleus of each breaks into two parts, a, 

 and b, of which a is retained and b handed over in exchange. 

 The old a and the new b then unite, and each paramcecium 

 goes on its separate way. M. Maupas, who has lately 

 reinvestigated this matter, considers, as the result of his 

 observations on another infusorian (Stylonichia) , that 

 without conjugation these organisms become exhausted, 

 and multiplication by fission comes to a standstill. If this 

 be so, conjugation is, in these organisms, necessary for the 

 continuance of the race. But Eichard Hertwig has recently 

 shown that this is, at any rate, not universally true. 



In the bell-animalcule, fission takes place in such a 

 manner as to divide the bell into two equal portions. 

 Thus there are two bells to one stalk. But the fate of the 



