4
CHR. P. RAVEN
They may be formed at the time of the maturation divisions, and again
at early cleavage divisions. In the first case they protrude at the moment
of the maturation divisions, but are afterwards incorporated again into
the egg. A polar lobe constricted off from the vegetal pole at first
cleavage, afterwards fuses with one of the cleavage cells, which thereby
becomes CD ; it then surpasses its sister cell AB in size by the volume of
the polar lobe. At the second cleavage, a similar polar lobe forms at CD,
which subsequently fuses with its daughter cell D. In this way the
substance of the polar lobe is assigned as a whole to one of the first four
blastomeres.
The corpuscular inclusions of the egg cytoplasm may be subjected to
considerable displacements during ooplasmic segregation. In many cases
the protein yolk granules, which were initially distributed more or less
evenly throughout the egg, are concentrated in the vegetal part of the
egg during or shortly after maturation, leaving an extensive yolk-free
area at the animal pole. This is most pronounced in the eggs of Opisthobranchiata (e.g.,Cymbulia:
Fol, 1875; Aplysia: Ries and Gersch, 1936;
Naranax: Worley and Worley, 1943), but it occurs to a lesser extent
also in some Prosobranchiata (e.g., Neritina:
Blochmann, 1882;
Columbella: Spek, 1934). This vegetal concentration of the yolk may
continue during cleavage, bringing about a still sharper separation of
egg substances. In general it leads to a distribution, in which the cells of
the first quartet contain hardly any protein yolk, those of the second
quartet somewhat more, and so on, while most of the yolk passes into
the endomeres.
A very unequal distribution of the yolk among the cells may already
have taken place at the first cleavages in some Opisthobranchiata, e.g.,
in Cymbulia and Cavolinia, where nearly all the yolk is found at the
4-cell stage in A, Β and C, whereas D consists almost exclusively of
hyaline protoplasm (Fol, 1875).
In the pulmonates Limnaea (Raven, 1945), Physa (Mancuso, 1953),
Succinea (Jura, 1960) and Arion (Lams, 1910) fat droplets are rather
uniformly distributed at first, but are shifted towards the vegetal part
of the egg during the maturation divisions. The same occurs in Bithynia
(Attardo, 1955a). Consequently the micromeres get only few fat droplets,
while most of the fat passes into the endomeres.
On the contrary, in the opisthobranchiates Aplysia (Ries and Gersch,
1936) and Navanax (Worley and Worley, 1943) the concentration of the
protein yolk in the vegetal two-thirds of the egg is accompanied by an
accumulation of the fatty yolk in the animal part, where it occupies a
layer between the protein yolk and the clear hyaloplasm area at the
animal pole. During cleavage it passes mainly into the micromeres.
In Ilyanassa the lipid droplets are likewise accumulated in the animal
CHR. P. RAVEN
They may be formed at the time of the maturation divisions, and again
at early cleavage divisions. In the first case they protrude at the moment
of the maturation divisions, but are afterwards incorporated again into
the egg. A polar lobe constricted off from the vegetal pole at first
cleavage, afterwards fuses with one of the cleavage cells, which thereby
becomes CD ; it then surpasses its sister cell AB in size by the volume of
the polar lobe. At the second cleavage, a similar polar lobe forms at CD,
which subsequently fuses with its daughter cell D. In this way the
substance of the polar lobe is assigned as a whole to one of the first four
blastomeres.
The corpuscular inclusions of the egg cytoplasm may be subjected to
considerable displacements during ooplasmic segregation. In many cases
the protein yolk granules, which were initially distributed more or less
evenly throughout the egg, are concentrated in the vegetal part of the
egg during or shortly after maturation, leaving an extensive yolk-free
area at the animal pole. This is most pronounced in the eggs of Opisthobranchiata (e.g.,Cymbulia:
Fol, 1875; Aplysia: Ries and Gersch, 1936;
Naranax: Worley and Worley, 1943), but it occurs to a lesser extent
also in some Prosobranchiata (e.g., Neritina:
Blochmann, 1882;
Columbella: Spek, 1934). This vegetal concentration of the yolk may
continue during cleavage, bringing about a still sharper separation of
egg substances. In general it leads to a distribution, in which the cells of
the first quartet contain hardly any protein yolk, those of the second
quartet somewhat more, and so on, while most of the yolk passes into
the endomeres.
A very unequal distribution of the yolk among the cells may already
have taken place at the first cleavages in some Opisthobranchiata, e.g.,
in Cymbulia and Cavolinia, where nearly all the yolk is found at the
4-cell stage in A, Β and C, whereas D consists almost exclusively of
hyaline protoplasm (Fol, 1875).
In the pulmonates Limnaea (Raven, 1945), Physa (Mancuso, 1953),
Succinea (Jura, 1960) and Arion (Lams, 1910) fat droplets are rather
uniformly distributed at first, but are shifted towards the vegetal part
of the egg during the maturation divisions. The same occurs in Bithynia
(Attardo, 1955a). Consequently the micromeres get only few fat droplets,
while most of the fat passes into the endomeres.
On the contrary, in the opisthobranchiates Aplysia (Ries and Gersch,
1936) and Navanax (Worley and Worley, 1943) the concentration of the
protein yolk in the vegetal two-thirds of the egg is accompanied by an
accumulation of the fatty yolk in the animal part, where it occupies a
layer between the protein yolk and the clear hyaloplasm area at the
animal pole. During cleavage it passes mainly into the micromeres.
In Ilyanassa the lipid droplets are likewise accumulated in the animal