BIGELOW : NUCLEAR CYCLE OF GONIONEMUS MUIIBACHII. 379 



VI. Summary. 



Somatic mitosis (Figs. 3-14). — The "resting" somatic nucleus of 

 Gouionemus is filled with dense karyoplasm ; there is an achromatic 

 reticulum with nodal thickenings or karyosomes, and a single large nu- 

 cleolus, consisting of a peripheral shell of chromatin and a central mass 

 of plasmatic substance. Many of the achromatic threads radiate from 

 the nucleolus. At this stage all portions of the nucleus except the nu- 

 cleolar shell combine with acid dyes, the latter combining with basic 

 ones. 



In the prophase the karyosomes increase in size, and reverse their 

 staining reaction simultaneously with the disappearance of the karyo- 

 plasm, while the nucleolar shell breaks down and contributes its sub- 

 stance to the chromatin structures. The karyosomes, together with the 

 masses of chromatin derived from the nucleolar shell, become concen- 

 trated along the courses of the achromatic strands, forming separate chro- 

 matin segments. These segments contract and form the dumb-bell-shaped 

 chromosomes without being metamorphosed into a continuous spireme 

 thread. 



The somatic number of chromosomes is probably twenty-four. 



No centrosome or archoplasmic structures are visible in the cell until 

 the metaphase, when the spindle figure is formed. This is extremely 

 simple, there being no trace of astral radiations. 



The centrosome is a minute granule at the focus of the spindle fibres. 

 After the anaphase it can no longer be detected. 



In the anaphase interzonal filaments are formed, which are stouter and 

 more clearly granular than the spindle fibres. 



In the reconstruction of the nucleus the nucleolus is formed by a con- 

 densation of chromatin granules. This body is at first entirely homo- 

 geneous, and only when it has attained its full size does the plasmatic 

 substance become differentiated in its central region. 



Spermatogonia (Figs. 15-3S). — In the spermatogenesis of Gouione- 

 mus the growth period takes place in the spermatogonia. 



Durinor the " resting " stage the nuclei are similar in structure and 

 staining reactions to those of somatic cells; the chromatin is diffused 

 and the nucleolus is of the compound type. The cytoplasm often con- 

 tains metaplasmic masses, but no archoplasm can be detected, and it is 

 doubtful whether a centrosome is present. 



In the early prophase the karyosomes, as in somatic cells, reverse 

 their staining reaction simultaneously with the disappearance of the 



