448 C. C. MACKLIN. 



appeared to have already commenced, viz., in which the nucleus 

 showed elongation and equatorial constriction, and observing 

 in detail the subsequent changes which it underwent. 



Such a cell presents an appearance of the following general 

 type. The nucleus is somewhat lengthened, and, in the zone 

 equidistant from its poles there is to be seen, on one or both 

 sides, an indentation. In this concavity is situated characteris- 

 tically a body, the centrosphere (fig. I, c) whose refractivity is 

 somewhat greater than that of the surrounding cytoplasm. Its 

 outline is indistinct, but the edge seems to be irregular with short 

 toothlike processes. These change their shape slowly, and give 

 the impression of being pushed out and drawn in very gradually. 

 They are intimately related to definite refractive bodies the 

 mitochondria which, often rodlike and sometimes threadlike in 

 form, radiate from the periphery of the centrosphere. Indeed 

 the movements of the latter (described by Lewis and Lewis '15) 

 may be responsible for the apparent movement in the periphery 

 of the centrosphere. 



Preparations fixed with osmic acid vapor and stained with 

 Heidenhain's iron hematoxylin (which was the method generally 

 employed) disclosed a minute granular body, generally paired, 

 within the centrosphere, w T hich is recognized as the centrosome 

 or centriole (Fig. 24). 



The position of the centrosphere within the nuclear concavity 

 or cleft has been noted by various authors, including Maximow 

 ('08), who found the centriole-pair thus situated in cells, the 

 nuclei of which appeared to be dividing directly, in the mesen- 

 chyme of the embryo rabbit. In the cells of tissue cultures 

 examined the centrosphere was absent from the cleft in only 

 two per cent, of cases, and in these exceptions it may have been 

 originally situated as in the others. 



OBSERVATIONS. 



A number of extended observations were made upon living 

 cells containing such elongated and constricted nuclei, and, as a 

 general rule, the nucleus rounded out again, assuming the usual 

 form. Thus it was demonstrated that constriction alone does 

 not indicate that the nucleus will divide directly. Finally, 



