BLACKMAN: SPERMATOGENESIS OF THE MYRIAPODS. 73 



integration of the nuclear membrane. It goes to the equator without 

 previous sign of division. 



Tetrad Formation. The typical process of tetrad formation in 

 insects, as described by Paulmier^*^ in Anasa and McClung'" in Ilip- 

 piscus, is as follows : The spireme splits lengthwise and segments 

 into a number of rods equal to the maturation number of chromo- 

 somes. These double rods shorten, become more finely granular, and 

 split again transversely. At this time the typical form is represented 

 by the cross-like figures shown in the plates ( figs. 5, 6, 7), although 

 there are many modifications. These cruciform masses condense, be- 

 come homogeneous, and stain more transparently. 



I have already described the process of tetrad formation in Scolo- 

 pendra. In the formation of the tetrads from the diffuse, flaky mass 

 of chromatin arising in the pseudo-germinal- vesicle stage there seems 

 to be no definite splitting of the segment, but the chromatin seems 

 to collect more densely at certain areas, and thus to form the tetrad 

 figures. 



As regards the sequence of the longitudinal and cross divisions of 

 the chromosomes, there seems to be much difference in the results of 

 various investigators on arthropods. Several observers, among whom 

 are Vom Rath,-" Paulmier,'^' and Montgomery j'^'^ describe the transverse 

 division as occurring first. Hacker-'^ and McClung,'-^ however, find 

 that the first is the equation and the second the reduction division. 

 McClung lays especial stress upon the importance of the later pro- 

 phase of the spermatocyte in determining the sequence of the follow- 

 ing divisions. His point seems to be very well taken, and, as in 

 Scolopendra, as well as in Ilippiscus, the longitudinal cleavage is 

 the first one made manifest in the prophase. I believe that the first 

 mitosis results in the longitudinal division of the tetrads. 



The Budding of the Nucleus. In the male germ-cell, nothing 

 has been described, I believe, which corresponds to that peculiar 

 structure which I have called the itomere. In the egg-cells of various 

 animals, however, a structure, similar in origin and, in some respects, 

 in function, has been reported by several investigators. This is the 

 yolk nucleus, as described by Blochmann, Scharff, Balbiani, and 

 others.-'" In the material upon which these investigators worked, the 

 yolk nucleus is derived from the chromatin reticulum of the nucleus 



18. Paulmier, F. C, 1889, loc. cit. 



19. McClung, C. E. 1900, loc. cit. 



20. Vom Rath, O., 1S92, loc. cit. 



21. Paulmier, F. C, 1899, loc. cit. 



22. Montgomery, T. H., jr., 1898 and 1901, loc. cit. 



23. Hacker, V., 1897; "Uebei- weitere Uebereinstimmungen zwischen den Fortpflanzungs- 

 vorgangen der Theire uiid Pflanzen." Biol. Cent., vol. 17. 



24. McClung, C. E., 1900, loc. cit. 



25. Wilson, E. B., 1900: "The Cell in Development and Inheritance." 



