blackman: the spermatogenesis of scolopendra. 



rial, but when studied closely a different interpretation must always 

 follow. In the spermatocytes of Scolopendra I believe it is impossible 

 to arrive at this conclusion, however strong a preconception the observer 

 may have had. The tetrad figures accompanying this article can by no 

 possibility be logically interpreted as representing anything but a longi- 

 tudinal, followed by a transverse division of the chromosome. In the 

 interpretation of the chromosomes of the first spermatocyte and in the 

 sequence of the succeeding divisions, I am gratified to note that P. Bouin, 

 working upon other species of Myriapoda, agrees with my conclusions for 

 Scolopendra. 



The forms of tetrad which are of most common occurrence in the 

 arthropods are modifications of the cross, double- V, and ring figures found 

 in Anasa (Paulmier, '98) and in Hippiscus (McClung, :00). It is 

 probable that the tetrads found in all of the other insects are obtained 

 by a greater or less modification of the same process. Such is evidently 

 the case in copepods (Riickert, '94; Hacker, '92), and in Gryllotalpa 

 (vom Rath, '92), and it seems also to be true of other invertebrates, — 

 Thalassema and Zirphea (Griffin, '99), Unio, (Lillie, '98), etc. 



The typical tetrad of arthropods, as exhibited in the Insecta (Hemip- 

 tera, Paulmier ; Orthropera, McClung) and in the Myriapoda (Scolo- 

 pendra, Blackman), is produced in the following manner : The chromatin 

 segments of the reduced number, as they arise from the spireme stage 

 (Insecta), or from the aggregated segments of the karyosphere (Myri- 

 apoda), are long slender threads of a granular character. Each thread 

 very quickly splits longitudinally, giving rise to two long slender seg- 

 ments extending parallel to each other. Very shortly after this longi- 

 tudinal split becomes visible, indications of the second division at right 

 angles to the first, may be seen. The further changes of the chromatin 

 segments in insects are not essentially different from those of Scolo- 

 pendra, which have already (p. 29) been described. 



While great unanimity exists among workers upon the germ cells of 

 invertebrates with regard to the nature of the tetrads, there is still con- 

 siderable dispute concerning the sequence of the divisions. By far the 

 greater number, however, agree that the longitudinal division comes 

 first and is succeeded by the cross division. Riickert, Hacker, McClung, 

 Blackman, and P. Bouin have arrived at this conclusion for arthropod-, 

 and Lee, Linville, Griffin, Klinckowstrom, and Francotte agree with 

 them for other invertebrates. The opposite view — i. e. that the reduc- 

 tion, or cross, division precedes — is held by the following: vom Rath, 

 Henking, Paulmier, and Montgomery for arthropods, and Lillie for 



7 



