310 Mr M c Doivall, A Preliminary Note, etc. 



They are rather elongated, sometimes rod-like, sometimes 

 more irregular in form. These chromosomes, however, do not 

 remain long in this condition. Even from their earliest appear- 

 ance they tend to lie in pairs, parallel to one another, though 

 there seems to be no actual union ; and very shortly they are seen 

 to be no longer parallel, but at an angle one to the other. 



Fusion between the approximating ends takes place, so that 

 six V-shaped double chromosomes are formed (Fig. 1). (This 

 figure is purely diagrammatic, and the chromosomes are somewhat 

 reduced in size. In no individual nucleus can the pairing be 

 seen in more than one or two cases.) 



A transitory contraction-phase seems to ensue. 



The V-shaped chromosomes then elongate considerably, and 

 form what at first sight appears to be a segmented spireme. 

 Sometimes an indistinct longitudinal split is visible in the chromo- 

 somes at this time (Fig. 2). 



Next, the chromosomes shorten and thicken, and again become 

 V-shaped ; a second contraction-phase seems to occur*. 



The angle of each V now points inwards towards the centre of 

 the nucleus, while the tips of the arms touch the nuclear mem- 

 brane. This gives the appearance of a secondary reticulum, with 

 a very dim network and twelve rounded nodal bodies, intensely 

 chromatic, of approximately equal size (Fig. 3). 



The next stage is interesting. The chromatin appears to run 

 more and more towards these false nodes, the arms of the V pro- 

 jecting inwards towards the angle becoming more and more 

 contracted, and the angle enclosed by them wider and wider ; so 

 that at last only the two nodal spots are left, now very close 

 together and enlarged, joined by a band of "Zwischen substanzf." 



We have thus twelve darkly-staining chromosomes of very 

 characteristic shape, fused together in pairs (Fig. 4). Frequently 

 there appears to be one very large pair, one slightly smaller, and 

 four a little smaller still. Often before the breaking-down of the 

 nuclear membrane a second split is visible, dividing each chromo- 

 some — probably the remains of the longitudinal split seen in the 

 pseudo-spireme stage — so that a tetrad appearance is found. 

 Meantime the nucleolus (N, diagrams 1 — 3), which has been 

 gradually diminishing in size, shrinks rapidly, and is finally 

 extruded into the cytoplasm just before the breaking down of the 



* Cf. Farmer and Moore, Q.J.M.S. Vol. 48, Part iv. 1905, p. 546. 



f K. Bonnevie {Anat. Anzeiger, Bd. xxvi. p. 501), describes this " Zwischen- 

 substanz " as chromatic in Enteroxenos : " Die beiden chromatinfaden der Doppel- 

 chromosoma sind, wie schon oben erwahnt, durch eine chromatische Zwischen- 

 substanz verbunden." But in the case of Angiostomum nigrovenosum it is 

 distinctly not chromatic, and apparently stains lightly with orange G. Perhaps, 

 however, "chromatische" is a misprint for " elastiche," which word is used in the 

 summary given at the end of the paper. 



