ALLEN— SEX INHERITANCE IN SPH.^ROCARPOS. 305 



chromosome on only one of the two homoeotypic spindles in a spore 

 mother cell, that in the heterotypic division the large and small chro- 

 mosomes pass respectively to different daughter nuclei. This means 

 that the separation of these two very different bodies is effected in 

 the same division as that which, on the basis of the best cytological 

 evidence, seems to result in the qualitative separation of other ("or- 

 dinary ") chromosomes. It remains to determine from a study of 

 the heterotypic division just how this separation is brought about. 



24 



■@ B 



Figs. 23 and 24. The two chromosome groups, seen in polar view, in a 

 metaphase stage of the homoeotypic division in a spore mother cell. The 

 great majority of the chromosomes on the two spindles (shown in Figs. 235 

 and 24 A) were in the same section; Fig. 23 A shows a single chromosome 

 belonging to the group of Fig. 23 B, appearing in the previous section ; and 

 Fig. 24 B, two chromosomes (or a dividing chromosome) belonging to the 

 group of Fig. 24 A, and appearing in the following section; x, the large 

 chromosome. X 3800. 



Of the four nuclei formed in the spore mother cell, two sister nuclei, 

 and therefore the two spores into which these nuclei pass, receive a 

 large chromosome each ; the other two nuclei (and spores) receive a 

 small chromosome each. Since the large chromosome is always 

 present in the cells of the female and never in those of the male, it 

 follows that a spore receiving a large chromosome must develop, if 

 it develops at all, into a female gametophyte, and that a spore re- 

 ceiving a small chromosome must develop, if it develops at all, into 

 a male gametophyte. 



The Chromosomes and Sex Characters. 



What I have hitherto referred to respectively as the "large" 

 and the " small " chromosome of Sphccrocarpos are so closely simi- 

 lar in appearance and behavior to the sex chromosomes of various 



PROC. AMER. PHIL. SOC . VOL, LVIII, T, AUG. H. I919. 



