206 Nichols, A morphological study of Juniperus communis var. depressa. 



pairing of the maternal and paternal elements of the chromatin, 

 followed during synapsis by their apparent fusion. Miss Ferguson 

 (1904) and Lewis (1908) are unable to find such procedure in the 

 microspore mother cells of Pinus and Thuja but maintain that 

 there the reticulum of the nucleus contracts directly into the 

 synaptic condition. Lewis finds occasional paired threads but 

 attaches no special significance to them. The writer's observations, 

 however, have led to the conclusion that in the species under dis- 

 cussion a presynaptic pairing does take place. 



The nuclei remain in synapsis for five or six days. Emerging 

 from it they undergo the heterotypic and homotypic divisions, and 

 within a week the tetrads have formed and have given rise to the 

 microspores. The ripening of the pollen takes about two weeks, 

 and pollination occurs about May 25 th . 



Toward the close of synapsis the tightly interwoven threads 

 begin to loosen up, and the mass of chromatic material gradually 

 resolves itself into a slender, somewhat roughened spirem of uni- 

 form thickness, which twists and coils throughout the nuclear cavity 

 (figs. 19, 20). The nucleus at this period bears a striking re- 

 semblance to the homologous stage asfigured in Drosera by Berghs 

 (1905, fig. 8), and no trace of the bivalent nature of the spirem 

 can be detected. As soon as the spirem has become completely 

 disentangled it begins to shorten and thicken, and it can be seen 

 in places that different portions of the thread are in contact 

 (fig. 21). Miss Ferguson (1904) finds that in Pinus neighboring 

 portions of the thread meet and fuse, and Noren (1907) states 

 that in J. communis the conditions are similar. That parts of the 

 spirem should touch one another is unavoidable, but a careful 

 examination has led to the conclusion that there is no actual 

 coalescence at the points of contact and that the chromosomes 

 arise, not from an "incompletely reticulated structure", but from 

 a continuous spirem. 



A longitudinal Splitting of the thick spirem soon becomes 

 apparent, and there seems to be no reason for not assuming that 

 the adjacent threads thus formed represent merely a reseparation 

 of the chromatic elements which have been actually or seemingiy 

 fused during synapsis. Almost simultaneously transverse breaks 

 appear, so that the spirem becomes divided up into a number of 

 double segments which are arranged end to end (fig. 22). Whether 

 the number of segments formed at this time corresponds with the 

 reduced number of chromosomes found during the later stages of 

 the heterotypic division cannot be stated with certainty, but sub- 

 sequent steps make this conclusion seem extremely probable. The 

 end to end arrangement of the segments is of short duration, and 

 the nucleus rapidly passes into the condition known as 'diakinesis', 

 the segments becoming variously oriented but for the most part 

 lying in the peripheral region of the nucleus (fig. 23). These 

 segments now represent the bivalent chromosomes of the hetero- 

 typic division, and their double nature can readily be made out in 

 well stained preparations. The two halves usually lie side by side 



