572 



THE AMEBIC Al 



RALIST [Vol. XL1II 



by the technical methods at our command. These difficulties, 

 however, have been clearly set forth by Mottier and other au- 

 thors, some of whom are unwilling to accept the hypothesis of 

 tlic permanence of the chromosome. 



Four papers have recently appeared which give further evi- 

 dence of the presence of prochromosomes in the resting nucleus 

 and also present some important conclusions on the history of 

 the chromatin during synapsis. In the latter feature these 

 authors (Overton, Lundegardh and Rosenberg) support the view 

 that during synapsis the sporophytic chromosomes by the 

 parallel association of two spirems become grouped in pairs to 

 form the reduced number of bivalent chromosomes character- 

 istic of the heterotypic mitosis. 



Overton 1 presents the results of studies on the pollen mother- 

 cells of Thalictrum purpurascens, Cahjcantltus florUhm and 

 Eichardia africana. He finds that the sporophytic (somatic) 

 nuclei previous to the heterotypic mitosis have their chromatin 

 in the form of definite bodies arranged in pairs with linin inter- 

 vals between. The bodies are prochromosomes and were traced 

 through synapsis to the chromosomes of the heterotypic mitosis. 

 Overton interprets the grouping of the prochromosomes in pairs 

 to mean that there are two spirems of paternal and maternal 

 origin in the sporophytic nuclei which he believes remain dis- 

 tinct throughout the sporophytic phase of the life history. The 

 parallel threads become more distinct just before synapsis and 

 become very closely associated during the synaptic contraction, 

 but remain distinct from one another. 



The association of the sporophytic chromosomes in pairs is 

 most intimate during postsynaptic stages when these elements 

 become more or less closely united in various ways to form the 

 bivalent chromosomes (in the reduced number) characteristic 

 of the heterotypic mitosis. This is the period in the life history 

 when the sporophytic chromosomes are most likely to influence 

 one another by conjugation or by the mutual interchange of 

 substance. 



The first or heterotypic mitosis in the pollen mother-cells dis- 

 tributes the sporophytic chromosomes associated in pairs. At 

 this time each sporophytic chromosome undergoes a longitudinal 



1 Overton, J. B., "On the Organization of the Nuclei in the Pollen 

 Mother-cells of Certain Plants, with Especial Reference to the Permanence 

 of the Chromosomes," Ann. of Bot., XXIII, p. 19, 1909. 



