210 



CYTOLOGY CHAP. 



different individuals. These chromosomes, however, are said to undergo 

 two longitudinal divisions in every mitosis. Each is said to divide 

 once during prophase, each daughter chromosome dividing again in the 

 metaphase. Another important point is that the spindle fibres do not 

 converge to a single centrosome, but run parallel with one another. 



Gamete formation (Fig. 89, D) begins with the emission of chromatin 

 particles from the nucleus into the cytoplasm. Each of these particles 

 becomes enclosed in a vesicle to form a minute secondary nucleus. 

 Closer examination shows that the chromatin particles which are emitted 

 from the original or primary nucleus are the individual "chromosomes" 

 which appear in the mitosis of this nucleus. The minute secondary 

 nuclei, each thus constituted out of a single " chromosome " of the 

 primary nucleus, multiply by repeated mitosis to form the gametes. 

 The number of chromosomes appearing in these mitoses is, however, 

 not one, but ten to twelve. 



Each of the enormous number of chromosomes of the primary nucleus 

 is therefore equivalent to an entire gamete nucleus containing ten to 

 twelve chromosomes. It may in fact be compared with the " un- 

 segmented spireme " found in the prophase of certain Metazoan and 

 Metaphytan mitoses (p. 9). 



The emission of the " chromosomes " of the primary nucleus into 

 the cytoplasm obviously suggests " chromidia formation," though in 

 this case it is merely the breaking up of a compound polyploid nucleus 

 into its constituent haploid (or diploid ?) nuclei, and therefore raises no 

 special problem. 



Amitosis, which, according to Borgert, occurs in Aulacantha in addition 

 to mitosis, also raises no difhculties in the case of a polyploid nucleus, 

 since each daughter nucleus may still have hundreds of representatives 

 of each individual chromatin element. 



Summing up, further knowledge is required before we can decide 

 whether nuclear multiphcation in the Protista by means of " amitosis " 

 and " chromidia " formation is to be conceived as an exception to, or 

 as a variant of,, the orderly division of the chromatin elements which 

 appears to be universal in the Metazoa and Metaphyta and to be at 

 least common in the Protista. 



B. ANIMALS AND PLANTS— HAPLOID AND DIPLOID 



CONDITIONS 



As will have been gathered from the occasional references to 

 plant cytology in the previous pages, the cytology of plants and animals 

 is on the whole so similar that detailed comparison is unnecessary. 

 Plants, however, exhibit a much more varied relation to the haploid 



