138 



SENESCENCE AND REJUVENESCENCE 



while in the anterior daughter animal a new mouth and pharynx 

 and probably a new oral groove arise (Fig. 42), while both mega- 

 nucleus and micronucleus' undergo division (Fig. 42), the process 

 in the former being apparently a direct or amitotic division, while 

 in the latter it resembles the process of mitosis in certain respects. 

 Before these divisions are completed a transverse constrictioii 

 appears at about the middle of the parent body (Fig. 42), and this 



mc 



o., 



o 



■0/ 



o ^ 



Figs. 41-43. — Three stages in the division of Paramecium: mc, micronucleus; 

 mg, meganucleus; og, oral groove; f, pharynx; », vacuoles of original individual; 

 v' , new vacuoles. 



deepens (Fig. 43), until finally separation of the two daughter 

 individuals occurs at this level. Before division occurs the cyto- 

 plasmic reorganization has reached an advanced stage (Fig. 42), 

 but the development of the oral groove and the attainment of the 

 characteristic proportions are not completed until after separation. 

 In Stentor coeruleus the process of agamic reproduction differs 

 in certain respects from that in Paramecium. In Stentor the first 

 visible stages in division are cytoplasmic, as in Paramecium, and 



' In the caudatam group of Paramecium only one micronucleus is present, while 

 in the aurelia group there are two. See Jennings and Hargitt ('lo), Woodruff ('ii6), 

 for the characteristics of these groups or species of Paramecium. 



