THE BIOLOGY OF THE CELL SURFACE 



form the basis for the extension of our knowledge. That 

 the clear, nearly homogeneous, almost structureless ground- 

 substance of an egg can (with a nucleus) develop, renders it 

 at once amenable to all treatment given an entire egg. As 

 it develops it exhibits the same form-changes. Here I 

 restrict myself to the discussion of one well-known phe- 



FiG. 3. — Boveri's classic figure of a mitotic spindle in the egg of Echinus 

 microtuberculatus. But cf. the aster in Fig. 29b (p. 174) and the mitotic complex 

 in Fig. 33d (p. 259). 



nomenon, namely, the appearance of the radiating struc- 

 tures seen in so many cells during cell-division. 



In many cells division of nucleus and of cytoplasm occur 

 simultaneously. Division of the nucleus may be a very 

 simple direct process, a constriction and separation of the 

 constricted parts or a more complex and indirect process 

 with the chromosomes going through a very orderly 

 sequence of manoeuvres on a bundle that appears to be 

 made up of threads. In this latter type of nuclear behavior 

 there often appear in the cytoplasm (occasionally in the 



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