BIGELOW: NUCLEAR CYCLE OF GONIONEMUS MURBACIIII. 301 



As the chromosomes approach the poles they hecome very closely 

 crowded together (Plate 2, Fig. 12), but at a slightly later stage they 

 separate once more and are now seen to be connected, probably at their 

 previous points of contact, so that they form an irregular network 

 (Fig. 13). At the same time the new nuclear membrane is formed in 

 each daughter cell, and constriction of the cell body takes place. Up to 

 this time the daughter nuclei remain connected with each other by the 

 bridge of interzonal filaments (Fig. 13), but these now break down and 

 are absorbed by the cytoplasm. With the further regressive meta- 

 morphosis of the nucleus the individual chromosomes disintegrate, so 

 that they are no longer distinguishable from one another, and the chro- 

 matic net becomes denser and more homogeneous. The latter then 

 breaks down into its minute karyosomes, which are connected by a deli- 

 cate limn reticulum and finally become dispersed throughout the nu- 

 cleus (Fig. 14). This stage is especially interesting from the light which 

 it throws on the question of the origin of the nucleolus. The two cells 

 represented in Figure 11 show the earliest stage at which this structure 

 is distinguishable from the karyosomes. It is now very small, hardly 

 larger than the latter, dense, and homogeneous, and in its staining prop- 

 erties, indistinguishable from chromatin. From its structure at this 

 stage it seems to me probable that it originates as the result of the coal- 

 escence of two or more karyosomes, just as it certainly does in the case 

 of the primary spermatocytes (page 311). During its growth it retains 

 its homogeneous character, and it is not until it reaches its mature size 

 (Plate 1, Fig. 3) that it shows any differentiation of its substance. 

 There can be little doubt, then, that the nucleolus of the somatic cell in 

 Gonionemus shows a very close genetic relationship to the chromatin 

 net ; or that its plasmatic central portion arises as a differentiation or 

 growth-product of the primitive chromatic mass. 



B. Spermatogenesis. 



The gonads of Gonionemus consist of a complex series of folds of the 

 oral surfaces of the radial canals, occupying the distal two-thirds of their 

 length. A cross section of the male gonad (Plate 1, Fig. 2) shows a 

 mass of sex cells which, as might be expected, are developed in the 

 ectoderm, and represent the several generations commonly described in 

 the spermatogenesis of other animals. Of these the spermatogonia lie 

 basally or next the mesogloea, while the more advanced generations 

 occupy a more superficial position. The arrangement, however, is rather 

 irregular. 



