chap, i.] CELLS. 7 



nuclear matter shows no definite boundary. The 

 nuclear membrane when present is a condensed outer 

 stratum of the nuclear matter. 



In some instances it can be shown that the 

 nuclear fibrils are in continuity with the fibrils of the 

 cell substance. In the moving white blood corpuscles 

 Strieker and linger have seen the nucleus becoming 

 one with the cell substance, and again afterwards 

 differentiated by the appearance of a membrane. 



8. During division of the cell the nucleus 

 generally divides before the cell protoplasm. This 

 division of the nucleus was until lately supposed 

 to occur in the same manner as that of the cell pro- 

 toplasm i.e., by simple cleavage. This mode is called 

 the direct division, or Remak's mode of division. 

 In this division the nucleus is supposed to become 

 constricted, kidney-shaped and hour-glass shaped, and 

 if the division is into more than two, lobed. Nuclei 

 of these shapes are not uncommon ; but they need not 

 necessarily indicate direct division, because, being 

 yery soft structures, pressure exerted from outside, or 

 the motion of the cell protoplasm, may produce these 

 shapes ; and, further, the contractility of the nucleus 

 may and occasionally has been observed to cause 

 these changes of shape. From the observations of 

 recent investigators Biitschli, Hertwig, Strassburger, 

 Mayzel, van Beneden, Balfour, Eberth, Schleicher, 

 Peremeschko, Flemming, Klein, Arnold, Pfiitzner, 

 Retzius, Bizzozero, and many others it is now known 

 that in the embryo and adult, in plant and animal, 

 vertebrates and invertebrates, all kinds of cells, before 

 their protoplasm undergoes division, show complicated 

 changes of their nucleus. This manner of division is 

 called the indirect division or karyokinesis. It has 

 been observed by Mayzel, Schleicher, and Flemming,. 

 that the nuclear fibrils show movement, hence the 

 name karyokinesis, i.e., "kernel-movement." This. 



