278 J. E. S. MOOEE. 



the same type of nuclei having gone down to the lumen of the 

 gland andj so to speak^ performed their dirty work them- 

 selves, travel back again to the more peripheral regions, pro- 

 ceeding by a series of extraordinary divisions to instal them- 

 selves directly as the nuclei of the eggs. During these 

 migrations the nuclei retain their peculiar character little 

 changed. In all the genital cells, the chromatin is aggregated 

 into one or two nucleoli (figs. 24, 30, 36), constituting a 

 nuclear type which represents the extreme term in a series, 

 whose mean would be represented by the intestinal nuclei of 

 Carcinus, Idotea, and others described by Frenzel -^ where the 

 chromatic substance is still, to a certain extent, distributed 

 through the mass of the nucleus, although some may be 

 aggregated into massive nucleoli. 



In Apus there is no vestige of colouring matter outside the 

 single chromosome which occupies its centre (fig. 36), the sub- 

 stance of which is so dense and refractive, that it appears like 

 a red lens suspended by one or two colourless threads from 

 the hollow sphere of the nuclear membrane. The intervening 

 space is entirely filled by a perfectly clear nuclearplasm. 



If such nuclei are at the periphery of the gland, and the egg 

 formation is about to begin, one of these single chromosomes 

 is seen to elongate just as the nucleolus in FrenzeFs '^Nucleo- 

 lare Kernbulbirung " become constricted in the middle, and 

 finally separate into two halves, the nuclear membrane being 

 but slightly elongated in the direction of the fission (figs. 

 36 — 41) . The two derived chromosomes may in turn divide at 

 right angles to the first separation axis, and a nucleus with 

 four chromosomes results (figs. 27, 29, 34, 35), whose mem- 

 brane is seen to be gradually tucked in at four intermediate 

 points, and at fig. 27 a final cross-shaped difi"erentiation can be 

 made out between the indentations. 



Along these lines the four quadrants ultimately separate, 

 giving rise to four nuclei, each with a single chromosome 

 (fig. 29). 



Such groups of four nuclei are always associated with the 

 ' ' Arch, fiir micro. Anat.,' Bd. xxxix. 



