ORIGIN OF THE TETRADS 



circular form. They finally condense into true tetrads which are 

 successively divided into dyads and monads by the two divisions ; 

 but it is an interesting fact that the order of division occurring in the 

 copepods appears here to be reversed, the first division being the 

 transverse and the second the longitudinal one a result agreeing 

 with Henking's earlier conclusion in the case of Pyrrochoris. Oster- 

 hout ('97) and Calkins ('97) independently discovered tetrads in the 

 vascular cryptogams (Equisetum, Pteris), and the last-named observer 

 finds that in Pteris they may arise either from rings, as in Gryllotalpa 

 or Heterocope, or from double rods as in Cyclops, the halves in the 

 latter case being either parallel or forming a cross. This longitu- 

 dinal split, occurring in the spireme, is followed by a transverse 

 division by which the tetrad is formed. Tetrads having an essentially 

 similar mode of origin are also described by Atkinson ('99) in Ari- 

 scema, and tetrad-formation is nearly approached in Allium according 

 to Ishikawa ('99). 1 These cases are considered at page 263. 



Resume. In all the foregoing cases the tetrads arise from a spi- 

 reme which splits lengthwise, segments into one-half the somatic 

 number of rods (each longitudinally divided) and each of the latter 

 divides transversely to form the tetrad. When the ends of the 

 daughter-chromosomes resulting from the longitudinal split remain 

 united (as in insects) ring-forms result, and the earlier phases of tetrad- 

 formation are thus identical with those of heterotypical mitosis. 

 When the split is complete, so that the ends remain free, double 

 rods result; while, if the daughter-chromosomes remain temporarily 

 united at the middle or at the end, X-, Y-, and V-shaped figures may 

 arise. In all these forms tetrad-formation is completed by the com- 

 plete separation of the daughter-rods, the transverse division of 

 each in the middle, and the condensation of the four resulting bodies 

 into a quadruple mass. As will be shown in Section C (p. 258) 

 the transverse division is in many forms delayed until after sepa- 

 ration of the longitudinal halves. In such cases no actual tetrads 

 are formed, though the result is the same. 



(b) Second Type. Tetrad-formation with two Longitudinal Divi- 

 sions. The only accurately known case of this type is Ascaris, the 

 object in which tetrads were first discovered by Van Beneden in 

 1883. Carnoy ('86, 2) reached the conclusion that the tetrads in 

 some other nematodes (OpJiiostomum, Ascaris clavata, A. lumbricoides} 

 arose by a double longitudinal splitting of the primary chromatin-rods. 



1 Vom Rath ('93, '59) has endeavoured to show that a process involving the formation of 

 true tetrads occurs in the salamander and the frog, but the later and more accurate studies 

 of Meves ('96) seem to leave little doubt that this was an error, and that the tetrads observed 

 in these forms are not of normal occurrence, as Flemming ('87) had earlier concluded. 

 Cf. p. 259. 



