432 WARREN H. LEWIS 



markedly homogeneous and transparent, having a refractive 

 index near to that of the cytoplasm, and often it can be iden- 

 tified only by the arrangement of the mitochondria and the 

 granules about it and of the nucleolus within. No nuclear 

 membrane is distinguishable, although the border of the nucleus 

 in the resting cell is clear and sharp. The nucleoli are, as a rule, 

 quite distinct, irregular in outline, and somewhat ragged. They 

 vary in size and usually each nucleus contains one or two large 

 ones and sometimes one or more small ones in addition. In the 

 normal living mesenchyme cell during prophase the chromo- 

 somes become visible and the nuclear outline is lost, but reap- 

 pears again after division during the reconstruction period. No 

 fibers can be seen in the clear nuclear spindle. The substance 

 of the spindle seems to come from the nucleus itself by what 

 appears to be a coagulation-like process resulting in the segrega- 

 tion of the chromatin material into the more highly refractive 

 chromosomes and their separation from the clear nuclear sap. 

 This nuclear sap disappears after division and the endosplasm 

 then closely surrounds the small mass of chromosomes. During 

 the reconstruction period the nucleus, or rather the chromo- 

 somes, which constitute the early daughter nuclei, swell slowly, 

 apparently by the imbibition of fluid, lose their highly refractive 

 condition, and become so closely packed together that the 

 nucleus appears almost homogeneous and the individual chro- 

 mosomes are no longer distinguishable. It is generally believed, 

 however, that they retain their individuality. Attention should 

 be called to the important supposition that the chromosomes 

 take up water or fluid during the reconstruction period and lose 

 fluid during the prophase stage of mitosis, because one of the 

 most marked effects of potassium permanganate was to bring 

 about shrinkage of the nucleus with expulsion of fluid vacuoles. 

 The cytoplasm appears to be more complex than the nucleus 

 and less homogeneous. The cytoplasm proper is usually regarded 

 as a homogeneous semifluid or semisolid substance, in which are 

 imbedded certain bodies or inclusions, namely, the mitochondria 

 and the granules. The cytoplasm, when examined under the 

 microscope in the living mesenchyme cells of our cultures, is 



