1902.] NATURAL SCIENCES OF PHILADELPHIA. 13 



gregarines, the method is to break up the appropriate host-organ on 

 a slide, add a drop of some fluid, aud place a cover-glass over the 

 mount. There is necessarily released a quantity of various organic 

 fluids, and these fluids are nearly always mucilaginous. That they 

 are responsible for certain of the phenomena displayed by grega- 

 rines is suggested by the following observations, which also bear 

 upon the question of gregarine progression. Fig. 12 shows a 

 gregarine distant a trifle more than its own length from a solid mass 

 of host-tissue. Between the gregarine and the host-tissue are a 

 number of small particles. If an animal so situated be watched, 

 it wiU be seen to advance slowly and imsteadily for a very short 

 distance, possibly the half of its length, but usually much less. It 

 will then stop, remain motionless for the fraction of a second, and 

 finally, with a sudden jerk, return to the position which it occupied 

 originally. The particles follow the movement of the gregarine, 

 those nearest to it moving the greatest distance. This suggests that 

 there is behind the gregarine a mass of an invisible, elastic substance, 

 in which both the gregarine aud the small particles are entangled. 

 As the animal advances, this elastic substance is stretched, aud 

 when the force which has caused the animal to advance is released, 

 it is brought back into its original position by the sudden shorten- 

 ing of the elastic substance. 



This phenomenon, which was seen time and again, first caused 

 me to question the truth of Schewiakoff ' s explanation of gregarine 

 progression. For, if the advance be due to the elongation of a 

 stalk behind the animal, this stalk should prevent the slipping back- 

 ward. As will be developed later, I believe gregarine progression 

 is due to slight muscular movements, not apparent under ordinary 

 observational conditions. In such cases as the one now under con- 

 sideration, the advance is resisted by the elastic sticky substance, 

 and when the power is released the gregarine is jerked passively 

 backward. Since it is those gregai'ines which are lying near the 

 host-tissue which behave in this manner, it is probable that the 

 elastic substance is derived in part from the host-cells. Gregarines 

 some distance from any host-tissue were never seen to act in this way. 



4. My studies had advanced to this point with no more result 

 than to conclude that Schewiakoff' s explanation of gregarine pro- 

 gression was probably incorrect. The case shown by fig. 12 sug- 

 gested that the problem was to be solved by watching the gregarine 



