6 



COMPARATIVE PHYSIOLOGY. 



sess several. The circular form may be regarded as the typical 

 form of both cells and nuclei, and their infinite variety in size 

 and form may be considered as in great part the result of the 

 action of mechanical forces, such as mutual pressure ; this is, of 

 course, more especially true of shape. Reduced to its greatest 

 simplicity, then, the cell may be simply a mass of protoplasm 

 with a nucleus. 



It seems probable that the numerous researches of recent 

 years and others now in progress will open up a new world of 



Fig. 1. — NucLBAn division. A-H, karyokinesisof atiesne-cell. A, nuclear reticnlum 

 in its ordinary state. B, preparing for division ; tlie contour is less defined, and 

 the fibers tiiicker and less intricate. C, wreath-stage ; ttie chromatin is arranged 

 m a complicated looping round the equator of the achromatin spindle. D, mo- 

 naster-stage ; the chromatin now appears as centripetal equatorial V's, each of 

 which should be represented as doable. E, a migration of the half of each chro- 

 matin loop towards opposite poles of the spindle. F, diaster-stage ; ■ the chroma- 

 tin forms a star, round each pole of a spindle, each aster being connected by 

 strands of achromatin. G, daughter-wreath stage ; the newly formed nuclei are 

 passing through their retrogressive development, which is completed in the rest^ 

 ing stage, H. c?-/, karyokinesis of an egg-cell, showing the smaller amount of 

 chromatin than in the tissue-cell. The stages rf, e,j^, correspond to D, E, P, re- 

 spectively. The polar star at the end of the spindle is composed of protoplasm- 

 granules of the cell itself, and must not be mistaken for the diaster (P). The 

 coarse lir.es represent the chromatin, the fine lines the achromatin, and the dotted 

 , lines cell-grannies. (Chiefly modified from Flemming.) X-Z, direct unclear divis- 

 ion in the cells of the embryonic integument of the European scorpion. After 

 Blochmann (Haddon). 



cell biology which will greatly advance our knowledge, espe- 

 cially in the direction of increased depth and accuracy. 



