THE BEHAVIOUR AND PHYSIOLOQY OF HERRING AND OTHER CLUPEIDS 269
1936; McMynn and Hoar, 1953; Outram, 1955). The rate of development is closely linked with temperature b e e Section 111, B); for
example at 14% gastrulation begins 22 hr after fertilization, whereas
at 11-S"C the eggs after 22 hr are still in the blastula stage. In the
following account no references are made to either time or temperature
scales.
Immediately after fertilization there is a rearrangement of the
cytoplasm and yolk. The blastula resulting from cleavage forms a cap
of small cells at the animal pole. Gastrulation then occurs. The cells of
the blastula lift slightly from the surface of the yolk, retaining contact
with it by a cellular " root " in the centre. Intucking occurs and first
a dorsal and then a ventral lip of the blastopore is formed. The
inwardly migrating cells from the lips meet to form a two-layered
embryo, and growth of the peripheral cells over the surface of the yolk
takes place. The blastopore closes when the migrating cells meet.
A head region is differentiated at the dorsal lip of the blastopore.
Mesoderm cells migrate to form a layer between the ectoderm and
endoderm, and segmentation then occurs. The embryo is about 2 mm
long at this stage. As segmentation continues the nervous system is
established and the rudiments of the sense organs (optic cups, olfactory
capsules and otic vesicles) are developed. The head and trunk segments
remain attached to the yolk, but the tail soon becomes free. The
embryo shows occasional movements. The eyes are formed but lack
pigment ; two otoliths are present in the otic vesicles. The gut tube is
formed and the heart commences beating (about 40 beats per min at
15°C). A dorsal aorta is present and a sub-intestinal vein; thc only
corpuscular elcmcntfi viAhlc in tho Mood arc a form of leucocytc!. 'I'hn
( y s then hctaornc! pigrncwtcd arid haLct1ing glunthi q y w a r on tho ht:wl.
l'hc gut is open posteriorly, but although tho Rtornodaeurn is viaiblct,
the gut is still closed antcriorly.
According to McMynn and Hoar (1 053) hatching of Clupcu p d l r ~ i i
was nearly always head first through a softened part of the chorion.
The development of the herring is not necessarily typical for
clupeids. Many of the species ( A h a , Clupeonella, Cmpiabsa) have a
much shorter incubation time and Yerceva (1939) and Lasker (1962)
have found that Cmpialosa caspia and Surdinope cuerulea larvae
respectively hatch with unpigmented eyes.
B. Effect of temperature on rate of development
Some of the published work is given in Table 11.
From the equation of Hela and Laevastu for herring the biological
zero (the theoretical temperature when incubation takes infinite tirnc)
1936; McMynn and Hoar, 1953; Outram, 1955). The rate of development is closely linked with temperature b e e Section 111, B); for
example at 14% gastrulation begins 22 hr after fertilization, whereas
at 11-S"C the eggs after 22 hr are still in the blastula stage. In the
following account no references are made to either time or temperature
scales.
Immediately after fertilization there is a rearrangement of the
cytoplasm and yolk. The blastula resulting from cleavage forms a cap
of small cells at the animal pole. Gastrulation then occurs. The cells of
the blastula lift slightly from the surface of the yolk, retaining contact
with it by a cellular " root " in the centre. Intucking occurs and first
a dorsal and then a ventral lip of the blastopore is formed. The
inwardly migrating cells from the lips meet to form a two-layered
embryo, and growth of the peripheral cells over the surface of the yolk
takes place. The blastopore closes when the migrating cells meet.
A head region is differentiated at the dorsal lip of the blastopore.
Mesoderm cells migrate to form a layer between the ectoderm and
endoderm, and segmentation then occurs. The embryo is about 2 mm
long at this stage. As segmentation continues the nervous system is
established and the rudiments of the sense organs (optic cups, olfactory
capsules and otic vesicles) are developed. The head and trunk segments
remain attached to the yolk, but the tail soon becomes free. The
embryo shows occasional movements. The eyes are formed but lack
pigment ; two otoliths are present in the otic vesicles. The gut tube is
formed and the heart commences beating (about 40 beats per min at
15°C). A dorsal aorta is present and a sub-intestinal vein; thc only
corpuscular elcmcntfi viAhlc in tho Mood arc a form of leucocytc!. 'I'hn
( y s then hctaornc! pigrncwtcd arid haLct1ing glunthi q y w a r on tho ht:wl.
l'hc gut is open posteriorly, but although tho Rtornodaeurn is viaiblct,
the gut is still closed antcriorly.
According to McMynn and Hoar (1 053) hatching of Clupcu p d l r ~ i i
was nearly always head first through a softened part of the chorion.
The development of the herring is not necessarily typical for
clupeids. Many of the species ( A h a , Clupeonella, Cmpiabsa) have a
much shorter incubation time and Yerceva (1939) and Lasker (1962)
have found that Cmpialosa caspia and Surdinope cuerulea larvae
respectively hatch with unpigmented eyes.
B. Effect of temperature on rate of development
Some of the published work is given in Table 11.
From the equation of Hela and Laevastu for herring the biological
zero (the theoretical temperature when incubation takes infinite tirnc)