* 

 NEW TYPES OF CHROMOSOME DISTRIBUTION. 125 



three dark bodies are sometimes present. Judging from the con- 

 ditions found in Fig. i, N and O, and from the relation of the 

 chromosomes and plasmosome in Prionidus, where it has been 

 fully worked out, it seems very probable that two of these dark 

 bodies may be the idiochromosomes and the other, the plasmo- 

 some. Fig. i, N and O, represent about the same stage in the 

 growth period, but are entirely different in appearance. In N, 

 the whole nucleolus stains intensely black, but again as shown 

 in 0, we see a pale plasmosome with the idiochromosomes em- 

 bedded in it. There is no doubt but that the nucleolus in both 

 cases represents the same thing. The only difference is in the 

 staining capacity of the plasmosome. What causes this differ- 

 ence is impossible to say. However, it is very probable that the 

 plasmosome is an active agent and as such is constantly under- 

 going changes, either chemical or physical, or both. In the 

 prophase of the first division (Fig. i, P) the plasmosome breaks 

 down and the idiochromosomes stand out plainly while the other 

 chromosomes are condensing. In the cases described by Wilson 

 ('05) the idiochromosomes were attached to a plasmosome and 

 not actually embedded in it as we find in Diplocodus. 



Fitchia spinosula Stal. 



In forms where the accessory chromosome is a single unpaired 

 element, this chromosome remains throughout the growth period 

 as a chromosome nucleolus, and since it divides in but one 

 maturation mitosis, half the spermatozoa receive one more 

 chromosome than the other. As already stated, these sperma- 

 tozoa produce females, the others males, since the female somatic 

 cells contain one more chromosome than the male. 



In Fitchia, an exactly similar result is produced, yet, the man- 

 ner in which the unequal distribution is brought about is entirely 

 different. The female cells (Fig. 2, A and B) have 28 chromo- 

 somes, and the male cells (Fig. 2, C and >), 27. In neither of 

 these groups can the differential chromosomes be recognized with 

 certainty. The first division (Fig. 2, E and F) shows 15 



period ; L, prophase of the first maturation division the plasmosome has 

 disappeared and the chromosomes are forming. The enlargement is 3,726 

 diameters. 



