216 



Prof. H. C. Bastian on Heterogenetic [Mar. 21 , 



cent parts. Gradually these particles became less sharply defined, and at 

 last scarcely visible, in the midst of a highly refractive protoplasmic mass 

 which began to exhibit traces of segmentation. 



Fig. 2. — Segmentation of Embryonal Areas into 

 Monads. (X16/0.) 



a. First stage of differentiation. 



b. Second stage ; area almost homogeneous and refractiye. 



c. First traces of segmentation. 



d. Segmentation more complete ; units highly refractive. 



e. Units less refractive, forming tailless corpuscles. 



/. Fully developed Monads derived from such corpuscles. 



Masses of this kind were seen which had been resolved by such a pro- 

 cess of segmentation into a number of spherical corpuscles about xroo" m 

 diameter. These were at first highly refractive, though they gradually 

 became rather less so, and revealed the presence of two or three minute 

 granules in their interior. In other adjacent areas, a number of densely 

 packed, pliant, and slightly larger corpuscles were seen actively pushing 

 against one another. When they separated they were found to be active 

 ovoid specimens of Monas lens about j^-/ in length, and provided with 

 a vacuole and a rapidly lashing flagellum. On the fourth day the number 

 of embryonal areas throughout the pellicle had increased, and the speci- 

 mens of Monas existed in myriads in the infusion. They were tolerably 

 uniform in size, though some were notably smaller than the average, owing 

 to the fact that they were products of a recent fission, all the stages 

 of which were watched on many occasions*. On the sixth day many of 



* It took place mostly in a longitudinal, though occasionally in a transverse direc- 

 tion. I have never seen the whole process occupy less than twenty minutes. 



