498 H. S. Jennings and George T. Hargitt. 



It was in 1883 that Maupas first set forth the distinctness 

 of the two species, based on the difference in the number and struc- 

 ture of the micro-nuclei in the two, as well as on more external 

 diversities. He says: ''All authors hitherto have described Para- 

 mecium as possessing only one nucleolus of rather large size, meas- 

 uring 0.005 to 0.008 mm. This is indeed the form most frequently 

 met. But I have observ^ed also numerous individuals provided 

 with two nucleoli, smaller and of different structure from the 

 preceding. They have a spherical form and are composed of a 

 central opaque corpuscle, staining deeply with dyes and measuring 

 only 0.003 mm., enveloped by a cortical layer 0.005 mm. in 

 diameter, transparent and not staining." (Maupas, '83, p. 660). 

 In his paper of 1888 he repeats this description of the difference 

 between the nucleoli, or micro-nuclei as he now calls them. He 

 adds that the number and structure of the micro-nuclei are the 

 distinctive and most important characters of the two species. 

 He did call attention to certain other differences; P. aurelia being 

 smaller with the posterior end rounded and having the two micro- 

 nuclei, while P. caudatum was a larger form with a single micro- 

 nucleus and a pointed posterior end. The first form, he says, 

 appeared always and only as P. aurelia, the latter always and only 

 as P. caudatum; the differences, both anatomical and physio- 

 logical, being sufficient to justify the distinction into two species. 



R. Hertwig ('89), in agreement with this distinction, used P. 

 aurelia for his study of the conjugation. He says, regarding the 

 micro-nuclei, that there are constantly two in this species but only 

 one in all other species. The two micro-nuclei are usually close to 

 each other and to the meganucleus, though commonly not enclosed 

 within a niche of the latter as was true in other species. Each 

 micro-nucleus, enclosed within a membrane, had a rather densely 

 staining nucleolus composed mainly of chromatin, and a cortical 

 part not staining. This is of course quite confirmatory of Maupas' 

 description of the structure of the micro-nuclei in the same species. 

 Both Maupas and Hert"vy;ig found the micro-nuclei to behave some- 

 what differently from those of P. caudatum in the conjugation 

 process. 



The work of these two investigators thus appeared to establish 



