20 



CYTOLOGY 



CHAP. 



coarser structure may contain larger chromatin masses which would not 

 receive that title. 



No general rule can be given as to the relation of the karyosomes of 

 the resting nucleus to the telophase or prophase chromosomes. In some 

 cases they are portions of the chromosomes which have failed to undergo 

 the telophase dissolution and remain as compact chromatin blocks. This 

 is well seen in Fig. 9 (Lepidosiren) . Here the anaphase chromosomes 



form a dense ring (daughter 

 plate), the apices of the V-shaped 

 chromosomes being on the inner 

 circumference of the ring, while 

 the limbs radiate outwards. At 

 the end of telophase, when the 

 daughter nucleus has been re- 

 constructed, a ring of karyosomes 

 occupies the place previously 

 occupied by the apices of the 

 Vs. These bodies gradually get 

 dispersed through the nucleus 

 and disappear, so that in the 

 middle of the resting period they 

 are absent. 



On the other hand, in many 

 forms in which karyosomes occur 

 they are not traceable back to 

 the telophase chromosomes, but 

 are secondary formations, the 

 newly reconstructed nucleus 

 being without them (A Ilium, 

 Lundegardh, 1913). 



As a rule, the number of 

 karyosomes in the nuclei of any 

 organism is highly variable, but 

 in other cases the number is 

 found to be constant, and to be the same as the number of chromo- 

 somes in the species in question, as shown by Rosenberg (1904). More- 

 over, these karyosomes may act as centres of formation for the 

 chromosomes in prophase, for which reason they have received the 

 name of prochromosomes (Overton, 1906). They have been specially 

 studied in plant cells (Fig. 10) . 



The presence of " prochromosomes " in the resting nucleus has been 

 taken, with some justice, as additional evidence of the continuity of the 

 chromosomes from one mitosis to another. It must be remembered, 



FIG. 9. 



Telophases in mesoderm cells of Lepidosiren. 

 A, side view ; B, polar view. 



