214 MAULSBY W. BLACKMAN. 



possess all the characteristics required of such an element. My 

 observations upon Scolopendra surely lend support to this theory. 

 Definite proof of the function of this structure can only be 

 obtained, however, by a study of the process occurring in the 

 fertilization and cleavage of the egg. 



My observations upon the accessory chromosome in Scolopen- 

 dra have added very little to our knowledge of this element, except 

 in so far as they help to show its wide distribution and the great 

 similarity of its behavior in widely separated groups. Indeed in 

 all important particulars the phenomena accompanying the devel- 

 opment of this structure are identical in Chilopoda and Orthoptera, 

 although the minor details of the process vary considerably. In 

 both groups the element is derived directly from a single sperma- 

 togonial chromosome, and for this reason takes no active part in 

 the phenomena of synapsis. During the prophase when the 

 other chromosomes divide into four chromatids and form tetrads, 

 this element, as would be expected from its origin, cleaves but 

 once and that longitudinally. In the two succeeding divisions 

 it is divided but once and thus is present in but one half of the 

 spermatids. The differences, although at times puzzling, are in 

 reality slight and unimportant. Thus, at the time when all of 

 the chromatin is aggregated in the karyosphere, the accessory 

 chromosome cannot be distinguished except in the most favor- 

 able cases ; but from the study of these thin, well-differentiated 

 'sections we are justified in saying that even in the pseudo-germ- 

 inal vesicle stage this element retains all its ordinary character- 

 istics. In the mataphase of the first spermatocyte it cannot be dis- 

 tinguished from the other chromosomes as it can in Orthopteran 

 material, because it is of approximately the same size as these. 

 In the second maturation division, however, it is again very evi- 

 dent, by reason of the fact that it divides longitudinally while the 

 other chromosomes divide transversely. 



These variations, as has been said, are unimportant modifications 

 of behavior and do not represent such fundamental differences as 

 seem to exist between the " small chromosome " (Paulmier) or the 

 "chromatin nucleolus " (Montgomery) in Hemiptera and the ac- 

 cessory chromosome in Orthoptera. If the observations of Paul- 

 mier and Montgomery concerning the origin of this element are 



