v of the Fishery Board for Scotland. 
293 
short stalk-like portion. If the unfertilised eggs so liberated are ripe, or 
nearly ripe, they quickly separate from each other, but if a mollusc is 
selected where the eggs are not mature, they remain adherent to each other 
for some time. In these irregular ovarian cells there is a large quantity of 
nutritive matter of a brown or brownish green, but not so much as always 
to obscure the clearer nucleus. The peripheral portions of these cells are 
somewhat clearer than the central parts. 
When, however, the ovary is of the characteristic brilliant reddish-brown 
colour already noticed, the artificially liberated eggs soon assume a regular 
spherical form (PL V. fig. 1). The ovum having assumed this symmetrical 
shape, the distribution of the deutoplasmic granules in the vitellus is more 
uniform. From the time of liberation of the ova till the redistribution of the 
deutoplasmic granules and the assumption of the spherical form, it takes, in 
the case of very mature cells, only a few minutes, but in other cases half an 
hour elapses between the polyhedral and perfectly spherical stages, the 
rate of alteration being perhaps also dependent on the temperature. 
In the ripe unfertilised and spherical ovum the nucleus (PI. V. fig. 1) is 
not so apparent as in the irregular polyhedral cell. This is due to the 
difference in distribution of the deutoplasmic granules, whose brownish- 
green colour completely obscures the nucleus. Notwithstanding, in some 
of those of a perfectly spherical shape, both a nucleus and nucleolus is 
distinctly seen. These, however, are the exception and not the rule. The 
vitelline membrane is clearly visible in both forms of ova. As in other 
Lamellibranch ova such as the common mussel,* Mytilus edulis, and 
Cardium pygmaeum,j at times outside of the bounding wall of the ovum, and 
enveloping it, one can see a hyaline investment, which, however, is so perfectly 
transparent that it is not easily made out. When the granules of deuto- 
plasm do not obscure the nucleus, this is seen to have a diameter rather 
greater than one-third of the diameter of the ovum. The nucleolus when 
seen is not sufficiently clear to reveal a regular chromatin structure, though 
there is an indication of differentation by the manifestation of a difference in 
the contents. The size of the ovum is '068 mm. When the testes are 
mature, the spermatozoa (PI. V. fig. 2) exude as a milky white fluid on 
the organs being punctured. When minced, and when some of the particles 
are transferred to a slide, along with the motile spermatozoa are found 
numerous spermatoblasts. The spermatoblasts (PI. V. fig. 3) are ovoid 
bodies, and when ready to liberate the spermatozoa the tails of the contained 
spermatozoa project from the surface and perform vibratile movements. 
The spermatoblasts exhibit certain clear vesicles — the mother sperm cells 
— and are found in abundance. Though the spermatozoa may, when 
liberated articially from the testes, be united in bundles, yet the heads 
quickly separate, and the individual spermatozoa move away from each 
other by the action of their tails and by any currents set up in the fluid 
under the cover glass. Amongst the specimens examined, it was now and 
again difficult to obtain individuals with ripe spermatozoa, the proportion of 
animals with ripe ovaries in some hauls of the dredge being greatly in excess. 
The spermatozoon differs slightly in shape from those of P. varius and 
P. glaber, as figured by Lacaze Duthiers.J The head is ovate, but some- 
what drawn out at the apex, presenting in fact an intermediate shape 
between the very pointed spermatozoon of the mussel as drawn by Wilson, § 
and the blunted apices of P. varius and P. glaber. The spermatozoon 
figured is magnified 1300 times. 
* Wilson, "On the Development of the Common Mussel," Fifth Annual Report 
of the Fishery Board for Scotland, p. 247, 1887. » 
tLoven, ' ' Bidrag til Kiinnetl. om Utweckl. af Moll. Acephala Lamellibr.," Vetensk. 
Akad. Handl., 1848 (Translation, Archiv.f. Naturg., 1849). 
X Loc. cit. § Luc. cit. , plate xii. lig. 33. 
