SPEEMATOGENESIS OF STENOBOTHEUS VIEIDTJLrS. 13 



and others. The heterotropic chromosome does not divide at this mitosis, 

 and may often be seen on its way to one pole, while the ordinary chromosomes 

 are still on the equatorial plate. Illustrations of this are shown on Plate 2. 

 figs. 21 & 22, and a still later stage in fig. 23, where the ordinary 

 chromosomes, still attached to one another by connecting-fibrils, are moving 

 towards the opposite poles. In every case the heterotropic chromosome 

 passes entire to one daughter cell, so that dimorphism of the spermatozoa is 

 effected at this mitosis, half the resulting secondarj'^ spermatocytes possessing 

 this odd chromosome, and half being without it. No resting-stage follows 

 the telophase, for the two maturation divisions occur in rapid succession. 



Secondaet Speematocytes. 



Large areas are frequently to be seen occupied by cells undergoing these 

 two divisions ; and the absence of an intervening resting-stage is not 

 characteristic only of this species, for it has been observed in many other 

 types. 



-The chromosomes that assemble on the mitotic spindle are nine or eight 

 in number, the difference depending upon the presence or absence of the 

 heterotropic chromosome, which is found in only half the cells. The complex 

 exhibits the same size relationship that occurred in the earlier metaphases : 

 there are once more three small chromosomes, of which two are spherical 

 and the third ovoid, three large chromosomes appearing as Vs with their 

 component arms closely folded on one another, and two chromosomes of 

 intermediate size, also represented by a pair of arms joined at one extremity. 

 The heterotropic chromosome, when present, is still the fourth largest of the 

 complex. Examples of polar views of the metaphase, showing the chromosome 

 complex, are given on Plate 2. figs. 25, 2Q, & 27. 



Later we see the ordinary chromosomes dividing in mitosis at the junction 

 of their component arms, one arm going to each pole. If the arms are 

 really the associated members of the spermatogonial pairs, this division 

 must effect the separation of chromosomes that became laterally associated 

 two generations previously ; and if we accept the further hypothesis that the 

 members of these pairs are respectively paternal and maternal in derivation, 

 we must regard this mitosis as the means of separating chromosomes 

 obtained from the two parents, after a possible exchange of chromatin. 

 There is however no direct evidence to prove that the component arms of 

 these V-shaped chromosomes correspond with the members of the spermato- 

 gonial pairs, or that the members of the pairs are derived from both 

 parents, although there are reasons for assuming the truth of these two 

 suppositions. 



The heterotropic chromosome is the last to divide, and can often be seen 

 on the periphery of the mitotic spindle when the ordinary chromosomes are 



