and Ordinary Forms of Primula sinensis. 241 



few test cases the vertical measure of the nucleus was estimated 

 by focussing on to the upper and lower walls of the nucleus by 

 means of the micrometer fine adjustment, taking the mean of 

 three readings. In each of these cases the measure so obtained 

 approximated very closely to those taken in the horizontal plane. 

 It is of course desirable that the tissues selected for com- 

 parison should be composed of cells which approach, as nearly as 

 may be, to uniformity of shape and size. Such tissues are to be 

 found in various epithelial layers, notably in the very regular 

 layer of cells which surrounds the embryo sac in the mature 

 unfertilized ovule. This layer is the " couche de revetement " of 

 Warming* and Vesquef, and the "tapetura" of Billings j, all of 

 whom have recorded its existence in species of Primula. Other 

 layers whose nuclei have been compared are the epidermis of the 

 young stigma as seen in approximately median longitudinal section, 

 and the epithelia of the young developing ovules. Equally favour- 

 able for purposes of comparison are the nuclei of the pollen mother 

 cells, since the individual cells at the same stage of development 

 are remarkably uniform in size, and the stages are marked by 

 easily recognizable characters of the nucleus, so that a just com- 

 parison can be made. I hoped to test the matter further by 

 measurements of the cells of the young embryos as they developed 

 within the seed, but owing to the resistance offered by the thicken- 

 ing integuments to the penetration of the fixing fluids I was unable 

 to obtain sufiiciently good material. The apex of the young 

 radicle may perhaps allow a further test, when it becomes available 

 later in the season. 



In dealing with the size of the chromosomes of the two forms, 

 I have had to be content with a comparison of the drawings made 

 with a camera lucida, since the chromosomes are too small to be 

 measured with any degree of accuracy by means of an ocular 

 micrometer. Figs. 1 and 2 show the groups of chromosomes of 

 the Ordinary and Giant forms, respectively, as they appear at 

 metaphase of the heterotype division. The figures represent the 

 view transverse to the spindle; the outlines of the chromosomes 

 were carefully drawn, and I think the difference between the two 

 is not exaggerated. Spindles seen in longitudinal view show that 

 the two forms had reached as nearly as possible identical stages; 

 a similar slight difference in the size of the chromosomes of the 

 two forms is apparent also in these longitudinal views. In the 

 somatic mitoses which I have seen the chromosomes were too 

 crowded to permit of fair comparison. 



* " De I'Ovule." Ann. des Sci. Nat., Ser. vi. Tome 5, p. 235, 1878. 

 t " Sur le Developpement du Sac embryonnaire." Ibid., Ser. vi. Tome 8, p. 360, 

 1879. 



J "Beitrage z. Kenntniss d. Samenentwickelung." Flora, 88, p. 277, 1901. 



