406 Delf . — 77 ^ Attaching Discs of the Ulvaceae. 
and two or three successive segments with usually more than one nucleus 
are cut off (PL XLV, Figs. 5, 6). It is probable that the small filaments 
bore their way into crevices and become firmly attached there, whilst the 
larger multinucleate segments give rise to the new thalli. All the filaments 
appeared to have been full of living contents at the time of fixing (early 
Text-fig. 3. Diagram of longitudinal section through disc of Ulva latissima , which has given 
rise to four fronds. The lines show the general direction of the filaments ; the cross-hatched region 
is part of the periphery of the disc which has not been cut in the median plane. 
spring), so that it is probable that, whilst the thallus of this species is 
annual, the discs are perennial. 
The nuclei vary somewhat in size, but always appear as small refractive 
bodies, with a single nucleolus. Two nuclei may often be seen closely 
associated, but no case of an actually dividing nucleus has been observed. 
Stained microtome sections through discs of Ulva , which were attached 
to a thallus of the Polysiphonia type, leave no doubt as to the parasitic or 
