142 EZRA ALLEN 



the same size are clearly present. In others, no large cross is 

 decipherable, but a small one appears. In two cells I saw one 

 large and one small cross. The metaphase complex in lateral 

 view shows a cross sometimes (fig. 57) and again none, but at this 

 stage the cross is very difficult to differentiate unless viewed en 

 face. Two are present in the cell shown in figure 27, a large and 

 a small one. This cell has sixteen rings (one not drawn), two 

 crosses, and one rod, the accessory. The curved rod is the con- 

 stant shape of the accessory after differentiation. (Figs. 27, 

 18, 19, 20, 21, 22, 54, 55, 56, and 58). 



The largest chromosome, A, shown in figures 27, 30-33, seems 

 variable between the double and triple form of ring. Its triple 

 form is often seen in the various prophase stages, and again, as in 

 figure 27, it is found in the double ring form, with a delicate 

 bridge of chromatin connecting the sides of the larger loop and a 

 more delicate one between the sides of the smaller loop (the upper 

 one in figure 27). Chromosome A in its triple form has been 

 noted four times in the same cell with chromosome B in double 

 ring form. The small double ring shown in figures 27 and 50 

 (from different rats) is very common. 



The sketch of the chromosomes appearing in figure 27 is, as 

 stated above, made from one cell. One chromosome, com- 

 pleting the full number 19, is not shown because its form is not 

 positively determinable. It appears like a double ring seen on end, 

 rather than a cross, and since no other shape but rings and crosses 

 (aside from the accessory) has been found in all the rats studied, 

 it is fair to assume that it is a ring lying in such a position as to 

 render its exact outline doubtful. This assumption is strength- 

 ened by the fact that in a near-by cell three double rings and 

 chromosome A in its triple form are present. In another, a 

 small double ring with widespread lugs is to be seen, which would 

 correspond in size with the one in question. It is drawn in 

 figure 28. It is not likely that this doubtful chromosome is A, 

 because it is too small, and, in addition, it has stained much more 

 deeply than A usually does at this period. Furthermpre, the 

 chromosome marked A in figure 27 is less dense than the others, 

 indicating in that respect its similarity to the normal condition 



