"ACCESSORY CHROMOSOME" IN EPEIRA. 197 



good ones it seemed conclusive that twenty-three was the correct 

 number. Owing to the fact that the chromosomes often lie one 

 upon the other, these counts were made with difficulty. In Fig. 

 i, b and c, distinct size and form differences may be readily ob- 

 served. These variations are due in part to foreshortening, yet 

 allowing for this error twenty-two of the chromosomes can be 

 symmetrically paired off into eleven pairs, while the twenty-third 

 is left without a mate. This is the small, round chromosome 

 marked 0. Since this chromosome has no mate, it can take no 

 part in synapsis, and I infer that it is the odd chromosome, which 

 persists as a compact, deeply-staining nucleolus throughout the 

 growth period, Fig. I, d-i. 



GROWTH PERIOD. 



Fig. i, d, represents the contraction phase of the early synaptic 

 period, with the ordinary chromosomes in the form of a closely 

 massed spireme, while the odd chromosome appears as a large 

 compact, deeply-staining nucleolus at one side of the contraction 

 figure. Even as early as this the odd chromosome often appears 

 double as in f, consisting of two closely approximated halves. 

 This I interpret as the result of an equation-division. This 

 double nature has been observed in other forms (cf. Syroinastcs, 

 Gross; Anasa, Wilson ; J3rachj'sto/a,Sutton; Acrididae, McClung). 

 In the spiders Wallace and Montgomery have described the two 

 accessory chromosomes as still separated at this period. In my 

 own preparations I find a few cells, of the early growth-period, 

 with two compact nucleoli, sometimes lying side by side and 

 sometimes widely separated. At first I thought one of these 

 might be a plasmosome, but with all of the differential stains 

 mentioned above, both bodies took the nuclear stain equally 

 well. These cells are of rare occurrence, and are, I think, 

 abnormalities, resulting from this early equation-division of the 

 odd chromosome. 



In a later stage (Fig. i, r) the chromosome threads loosen, and 

 appear as longitudinally-split rods, with knobbed ends, the odd 

 chromosome here being often bipartite in appearance. These 

 chromosome threads shorten and thicken to form the chromo- 

 somes of the late growth period. In g and h the eleven ordinary 



