154 



INTRODUCTION TO CYTOLOGY 



not disappear but remains visible in the cytoplasm through the ensuing 

 metabolic stage. They concluded that the centrosome, like the nucleus, 

 is a permanent organ maintaining its individuality throughout successive 

 cell generations. This conclusion is supported by the conditions observ- 

 able in many tissues, but it is evidently not valid for all. In certain 

 instances it disappears at the close of cell-division, a new one appearing 

 just before the next mitosis. Moreover, the formation of numerous 

 asters with centrosomes can be induced in the cytoplasm of certain 

 animal eggs, notably those of echinoderms, by treating them with solu- 





W» 



<??= 





Fig. 83. — Centrosomes in epi- 

 thelium of cornea of monkey (A) 

 and of human gastric gland (B). 

 {After A. Zimmermann.) 



Fia. 





84. — Artificial cytasters in egg of Arhacia. 

 {After Morgan, 1899.) 



tion of various chlorides^ (Fig. 84). Such "artificial cytasters" were 

 thought by some to be quite distinct from normal ones in lacking the 

 ability to function in mitosis, but it now seems clear that they do have 

 this ability, and hence that functional centrosomes can be differentiated 

 independently of preexisting ones.^ Tharaldsen finds that, although 

 they may arise anywhere in the cytoplasm in Asterias eggs, the region 

 near the nucleus is "most susceptible," and that cytasters arising here are 

 usually dominant in the mitotic activity which follows. This recalls 

 Yatsu's observation that in Cerebratulus the cytasters do not appear 

 unless the nuclear boundary has begun to break down in late prophase. 



6 Mead (1898), Morgan (1896, 1899), Wilson (1901), Yatsu (1905), and later 

 workers. See Wilson (1925; p. 684). 



« Tharaldsen (1926), Bataillon (1929), Fankhauser (1929). 



