78 



CYTOLOGY 



CHAP. 



of the early cleavage stages are double, each portion being the direct 

 descendant of one of the gamete nuclei. Each constituent of such a 

 double nucleus is called a gonomere. In prophase each gonomere forms its 

 chromosomes separately from the other. The two groups of chromosomes 

 thus formed are usually indistinguishable from one another after the 

 break-down of the nuclear membrane, but in telophase they become 

 recognizable again owing to the fact that the group of chromosomes 

 derived from each gonomere again forms a nucleus distinct from, though 

 closely apphed to, that formed by the other group. Occasionally, however, 

 the two groups are distinct during metaphase and anaphase as well 

 (Fig. 35, A). In later cleavage stages the chromatin derived from the two 

 gamete nuclei gradually mingles more and more, and double nuclei 

 become consequently rarer. In Cyclops brevicornis double and bilobed 



' Fig. 35. 



Gonomerv in Cyclops ^trenuus. (After Riickert. A.m.A., 1895.) A, 2-4-cell stage the groups of chromo- 

 ^i^-tZl from i and 9 -ametes quite separate; B, 4-ceU stage. In the nucleus m prophase the two 

 ^"up\ o7chromormi"t sin C 3 -ceU sta|e. Gonomeres indicated in most of the nucle. 



nuclei are still common in the 64-cell stage, and in later stages bilobed 

 nuclei with a nucleolus in each lobe are still to be found, as well as spherical 

 nuclei with two symmetricaUy placed nucleoh, which Hacker interprets 

 as the last remaining indication of gonomery. In the germ-track (see 

 p. 79) evidences of gonomery can be found at a much later stage of 

 development than in the somatic cells (Hacker, 1903). 



A remarkable instance of gonomery is to be found in the Protozoan, 

 Amoeba diploidea (Nagler, 1909). This animal possesses two nuclei, in 

 close apposition to one another (Fig. 36), exactly like the double nuclei 

 of early Cyclops embryos. The hfe history shows that these nuclei are 

 the direct descendants of the two gamete nuclei— ^.^. they are gonomeres. 

 During the asexual reproduction of the animal the two nuclei divide 

 separately, but simultaneously, so that each daughter cell again receives 

 a double nucleus. Sexual reproduction begins by the coming together 

 of two individuals which enclose themselves in a common cyst. Now 

 in each individual the gonomeres for the first time fuse into a zygote 



