Chromosomes in Germ-Cells of Culex 215 



(Randolph '08) in which an equal pair of chromosomes has been 

 described as resembling the odd heterochromosome and the un- 

 equal heterochromosome-pair of other insects, in that it remains 

 condensed during the growth stage of the spermatocytes. The 

 conditions in Culex would indicate that in other cases where no 

 heterochromosomes have been found, they may nevertheless be 

 present, combined with other chromosomes in such a way that a 

 slight inequality in size may easily escape detection, and that 

 their characteristic behavior during the growth stage of the sperma- 

 tocytes may be changed by the influence of the pair with which 

 they are combined. As to the relation of the heterochromosomes 

 to sex determination further discussion seems to be of little value 

 until we get more evidence in connection with experimental 

 breeding. 



Synizesis 



The synizesis stages of Culex apparently have no relation what- 

 ever to the phenomena of synapsis, but are interesting in that 

 they afford further evidence that synizesis is a period of recon- 

 struction in which certain elements present in the spermatogonia! 

 chromosomes are either rejected as w^aste material or are isolated 

 in order that they may perform some function in connection with 

 the growth stages of the germ-cells. 



Synapsis 



Perhaps the most interesting point in the history of the germ- 

 cells of Culex is the fact that, as in the Muscidae, pairing, or syn- 

 apsis, occurs in connection with each spermatogonia! and oogon- 

 ial mitosis as well as in anticipation of maturation. I have not 

 been able to study somatic mitoses in Culex, but in the Muscidae 

 a similar pairing was found in follicle cells of the ovaries, and it 

 may therefore be true that pairing of homologous chromosomes 

 occurs in connection with each mitosis throughout the life history 

 of these insects, as Overton thinks probable in the case of sev- 

 eral of the higher flowering plants whose cytology he has studied. 

 Both in the Muscidae and in Culex, all the evidence indicates 



